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FAQ: Automotive Gasoline
Bruce Hamilton
B.Hamilton_at_irl.cri.nz
_________________________________________________________________
Subject: 1. Introduction and Intent
The intent of this FAQ is to provide some basic information on
gasolines and other fuels for spark ignition engines used in
automobiles. The toxicity and environmental reasons for recent and
planned future changes to gasoline are discussed, along with recent
and proposed changes in composition of gasoline. This FAQ intended to
help readers choose the most appropriate fuel for vehicles, assist
with the diagnosis of fuel-related problems, and to understand the
significance of most gasoline properties listed in fuel
specifications. I make no apologies for the fairly heavy emphasis on
chemistry, it is the only sensible way to describe the oxidation of
hydrocarbon fuels to produce energy, water, and carbon dioxide.
Subject: 2. Table of Contents
- Introduction and Intent
- Table of Contents
- What Advantage will I gain from reading this FAQ?
- What is Gasoline?
4.1 Where does crude oil come from?.
4.2 When will we run out of crude oil?.
4.3 What is the history of gasoline?
4.4 What are the hydrocarbons in gasoline?
4.5 What are oxygenates?
4.6 Why were alkyl lead compounds added?
4.7 Why not use other organometallic compounds?
4.8 What do the refining processes do?
4.9 What energy is released when gasoline is burned?
4.10 What are the gasoline specifications?
4.11 What are the effects of the specified fuel properties?
4.12 Are brands different?
4.13 What is a typical composition?
4.14 Is gasoline toxic or carcinogenic?
4.15 Is unleaded gasoline more toxic than leaded?
- Why is Gasoline Composition Changing?
5.1 Why pick on cars and gasoline?
5.2 Why are there seasonal changes?
5.3 Why were alkyl lead compounds removed?
5.4 Why are evaporative emissions a problem?
5.5 Why control tailpipe emissions?
5.6 Why do exhaust catalysts influence fuel composition?
5.7 Why are "cold start" emissions so important?
5.8 When will the emissions be "clean enough"?
5.9 Why are only some gasoline compounds restricted?
5.10 What does "renewable" fuel/oxygenate mean?
5.11 Will oxygenated gasoline damage my vehicle?
5.12 What does "reactivity" of emissions mean?
5.13 What are "carbonyl" compounds?
5.14 What are "gross polluters"?
- What do Fuel Octane ratings really indicate?
6.1 Who invented Octane Ratings?
6.2 Why do we need Octane Ratings?
6.3 What fuel property does the Octane Rating measure?
6.4 Why are two ratings used to obtain the pump rating?
6.5 What does the Motor Octane rating measure?
6.6 What does the Research Octane rating measure?
6.7 Why is the difference called "sensitivity"?
6.8 What sort of engine is used to rate fuels?
6.9 How is the Octane rating determined?
6.10 What is the Octane Distribution of the fuel?
6.11 What is a "delta Research Octane number"?
6.12 How do other fuel properties affect octane?
6.13 Can higher octane fuels give me more power?
6.14 Does low octane fuel increase engine wear?
6.15 Can I mix different octane fuel grades?
6.16 What happens if I use the wrong octane fuel?
6.17 Can I tune the engine to use another octane fuel?
6.18 How can I increase the fuel octane?
6.19 Are aviation gasoline octane numbers comparable?
- What parameters determine octane requirement?
7.1 What is the effect of Compression ratio?
7.2 What is the effect of changing the air/fuel ratio?
7.3 What is the effect of changing the ignition timing
7.4 What is the effect of engine management systems?
7.5 What is the effect of temperature and Load?
7.6 What is the effect of engine speed?
7.7 What is the effect of engine deposits?
7.8 What is the Road octane requirement of an vehicle?
7.9 What is the effect of air temperature?.
7.10 What is the effect of altitude?.
7.11 What is the effect of humidity?.
7.12 What does water injection achieve?.
- How can I identify and cure other fuel-related problems?
8.1 What causes an empty fuel tank?
8.2 Is knock the only abnormal combustion problem?
8.3 Can I prevent carburetter icing?
8.4 Should I store fuel to avoid the oxygenate season?
8.5 Can I improve fuel economy by using quality gasolines?
8.6 What is "stale" fuel, and should I use it?
8.7 How can I remove water in the fuel tank?
8.8 Can I use unleaded on older vehicles?
- Alternative Fuels and Additives
9.1 Do fuel additives work?
9.2 Can a quality fuel help a sick engine?
9.3 What are the advantages of alcohols and ethers?
9.4 Why are CNG and LPG considered "cleaner" fuels.
9.5 Why are hydrogen-powered cars not available?
9.6 What are "fuel cells" ?
9.7 What is a "hybrid" vehicle?
9.8 What about other alternative fuels?
9.9 What about alternative oxidants?
- Historical Legends
10.1 The myth of Triptane
10.2 From Honda Civic to Formula 1 winner.
- References
11.1 Books and Research Papers
11.2 Suggested Further Reading
Subject: 3. What Advantage will I gain from reading this FAQ?
This FAQ is intended to provide a fairly technical description of what
gasoline contains, how it is specified, and how the properties affect
the performance in your vehicle. The regulations governing gasoline
have changed, and are continuing to change. These changes have made
much of the traditional lore about gasoline obsolete. Motorists may
wish to understand a little more about gasoline to ensure they obtain
the best value, and the most appropriate fuel for their vehicle. There
is no point in prematurely destroying your second most expensive
purchase by using unsuitable fuel, just as there is no point in
wasting hard-earned money on higher octane fuel that your automobile
can not utilize. Note that this FAQ does not discuss the relative
advantages of specific brands of gasolines, it is only intended to
discuss the generic properties of gasolines.
Subject: 4. What is Gasoline?
4.1 Where does crude oil come from?.
The generally-accepted origin of crude oil is from plant life up to 3
billion years ago, but predominantly from 100 to 600 million years ago
[1]. "Dead vegetarian dino dinner" is more correct than "dead dinos".
The molecular structure of the hydrocarbons and other compounds
present in fossil fuels can be linked to the leaf waxes and other
plant molecules of marine and terrestrial plants believed to exist
during that era. There are various biogenic marker chemicals such as
isoprenoids from terpenes, porphyrins and aromatics from natural
pigments, pristane and phytane from the hydrolysis of chlorophyll, and
normal alkanes from waxes, whose size and shape can not be explained
by known geological processes [2]. The presence of optical activity
and the carbon isotopic ratios also indicate a biological origin [3].
There is another hypothesis that suggests crude oil is derived from
methane from the earth's interior. The current main proponent of this
abiotic theory is Thomas Gold, however abiotic and extraterrestrial
origins for fossil fuels were also considered at the turn of the
century, and were discarded then.
4.2 When will we run out of crude oil?
It has been estimated that the planet contains over 1.4 x 10^15 tonnes
of petroleum, however much of this is too dilute or inaccessible for
current technology to recover [4]. The petroleum industry uses a
measure called the Reserves/Production ratio (R/P) to monitor how
production and exploration are linked. This is based on the concept of
"proved" reserves of crude oil, which are generally taken to be those
quantities which geological and engineering information indicate with
reasonable certainty can be recovered in the future from known
reservoirs under existing economic and operating conditions. The
Reserves/Production ratio is the above reserves divided by the
production in the last year, and the result is the length of time that
those remaining reserves would last if production were to continue at
the current level [5]. It is important to note those definitions, as
the price of oil increases, marginal fields become "proved reserves",
thus we are unlikely to "run out" of oil, as more fields will become
economic as the price rises. If the price exceeds $30/bbl then
alternative fuels may become competitive, and at $50-60/bbl
coal-derived liquid fuels are economic, as are many biomass-derived
fuels and other energy sources [6]. One barrel of oil equals 0.158987
m3. The current price for Brent Crude is approx. $18/bbl. The R/P
ratio has increased from 27 years (1979) to 43.1 years (1993) [5].
Now, some numbers.
( billion = 1 x 10^9. trillion = 1 x 10^12 ).
Crude Oil Proved Reserves R/P Ratio
Middle East 89.6 billion tonnes 95.1 year
USA 4.0 9.9 years
Total World 136.7 43.1 years
Coal Proved Reserves R/P Ratio
USA 240.56 billion tonnes 267 years
Total World 1,039.182 236 years
Natural Gas Proved Reserves R/P Ratio
USA 4.7 trillion cubic metres 8.8 years
Total World 142.0 64.9 years.
4.3 What is the history of gasoline?
In the late 19th Century the most suitable fuels for the automobile
were coal tar distillates and the lighter fractions from the
distillation of crude oil. During the early 20th Century the oil
companies were producing gasoline as a simple distillate from
petroleum, but the automotive engines were rapidly being improved and
required a more suitable fuel. During the 1910s, laws prohibited the
storage of gasolines on residential properties, so Charles F.
Kettering ( yes - he of ignition system fame ) modified an IC engine
to run on kerosine. However the kerosine-fuelled engine would "knock"
and crack the cylinder head and pistons. He assigned Thomas Midgley
Jr. to confirm that the cause was from the kerosine droplets
vaporising on combustion as they presumed . Midgley demonstrated that
the knock was caused by a rapid rise in pressure after ignition, not
during preignition as believed [7]. This then lead to the long search
for anti-knock agents, culminating in tetra ethyl lead [8]. Typical
mid-1920s gasolines were 40 - 60 Octane [9].
Because sulfur in gasoline inhibited the octane-enhancing effect of
the alkyl lead, the sulfur content of the thermally-cracked refinery
streams for gasolines was restricted. By the 1930s, the petroleum
industry had determined that the larger hydrocarbon molecules
(kerosine) had major adverse effects on the octane of gasoline, and
were developing consistent specifications for desired properties. By
the 1940s catalytic cracking was introduced, and gasoline compositions
became fairly consistent between brands during the various seasons.
The 1950s saw the start of the increase of the compression ratio,
requiring higher octane fuels. Lead levels were increased, and some
new refining processes ( such as hydrocracking ), specifically
designed to provide hydrocarbons components with good lead response
and octane, were introduced. Minor improvements were made to gasoline
formulations to improve yields and octane until the 1970s - when
unleaded fuels were introduced to protect the exhaust catalysts that
were also being introduced for environmental reasons. From 1970 until
1990 gasolines were slowly changed as lead was phased out. In 1990 the
Clean Air Act started forcing major compositional changes on gasoline,
and these changes will continue into the 21st Century because gasoline
is a major pollution source.
4.4 What are the hydrocarbons in gasoline?
Hydrocarbons ( HCs ) are any molecules that just contain hydrogen and
carbon, both of which are fuel molecules that can be burnt ( oxidised
) to form water ( H2O ) or carbon dioxide ( CO2 ). If the combustion
is not complete, carbon monoxide ( CO ) may be formed. As CO can be
burnt to produce CO2, it is also a fuel.
The way the hydrogen and carbons hold hands determines which
hydrocarbon family they belong to. If they only hold one hand they are
called "saturated hydrocarbons" because they can not absorb additional
hydrogen. If the carbons hold two hands they are called "unsaturated
hydrocarbons" because they can be converted into "saturated
hydrocarbons" by the addition of hydrogen to the double bond.
Hydrogens are omitted from the following, but if you remember C = 4
hands, H = 1 hand, and O = 2 hands, you can draw the full structures
of most HCs.
Gasoline contains over 500 hydrocarbons that may have between 3 to 12
carbons, and gasoline used to have a boiling range from 30C to 220C at
atmospheric pressure. The boiling range is narrowing as the initial
boiling point is increasing, and the final boiling point is
decreasing, both changes are for environmental reasons. Detailed
descriptions of structures can be found in any chemical or petroleum
text discussing gasolines [10].
4.4.1 Saturated hydrocarbons ( aka paraffins, alkanes )
- stable, the major component of gasolines
- tend to burn in air with a clean flame
alkanes
normal = continuous chain of carbons ( Cn H2n+2 )
normal heptane C-C-C-C-C-C-C C7H16
iso = branched chain of carbons ( Cn H2n+2 )
iso octane = C C
( aka 2,2,4-trimethylpentane ) | |
C-C-C-C-C C8H18
|
C
cyclic = circle of carbons ( Cn H2n )
( aka Naphthenes )
cyclohexane = C
/ \
C C
| | C6H12
C C
\ /
C
4.4.2 Unsaturated Hydrocarbons
- Unstable, are the remaining component of gasoline.
- Tend to burn in air with a smoky flame.
Alkenes ( aka olefins, have carbon=carbon double bonds )
These are unstable, and are usually limited to a few %.
C
| C5H10
2-methyl-2-butene C-C=C-C
Alkynes ( aka acetylenes, have carbon-carbon triple bonds )
These are even more unstable, are only present in trace amounts, and
only in some poorly-refined gasolines.
_
Acetylene C=C C2H2
Arenes ( aka aromatics )
Used to be up to 40%, gradually being reduced to
C C
// \ // \
C C C-C C
Benzene | || Toluene | ||
C C C C
\\ / \\ /
C C
C6H6 C7H8
Polynuclear Aromatics ( aka PNAs or PAHs )
These are high boiling, and are only present in small amounts in
gasoline. They contain benzene rings joined together, and the simplest
is Naphthalene. The multi-ringed PNAs are highly toxic, and are not
present in gasoline.
C C
// \ / \\
C C C
Naphthalene | || | C10H8
C C C
\\ / \ //
C C
4.5 What are oxygenates?
Oxygenates are just preused hydrocarbons :-). They contain oxygen,
which can not provide energy, but their structure provides a
reasonable anti-knock value, thus they are good substitutes for
aromatics, and they may also reduce the smog-forming tendencies of the
exhaust gases [11].
Ethanol C-C-O-H C2H5OH
C
|
Methyl tertiary butyl ether C-C-O-C C4H90CH3
(aka tertiary butyl methyl ether ) |
C
They can be produced from fossil fuels eg methanol (MeOH), methyl
tertiary butyl ether (MTBE), tertiary amyl methyl ether (TAME), or
from biomass, eg ethanol(EtOH), ethyl tertiary butyl ether (ETBE)).
Most oxygenates used in gasolines are either alcohols ( Cx-O-H ) or
ethers (Cx-O-Cy), and contain 1 to 6 carbons. MTBE is produced by
reacting methanol ( from natural gas ) with isobutylene in the liquid
phase over an acidic ion-exchange resin catalyst at 100C. The
isobutylene was initially from refinery catalytic crackers or
petrochemical olefin plants, but these days larger plants produce it
from butanes. Production has increased at the rate of 10 to 20% per
year, and the spot market price in June 1993 was around $270/tonne
[11]. The "ether" starting fluids for vehicles are usually diethyl
ether ( liquid ) or dimethyl ether ( aerosol ). Note that " petroleum
ether " is actually a volatile hydrocarbon fraction, it is not a
Cx-O-Cy compound.
Oxygenates are added to gasolines to reduce the reactivity of
emissions, but they are only effective if the hydrocarbon fractions
are carefully modified to utilise the octane and volatility properties
of the oxygenates. If the hydrocarbon fraction is not correctly
modified, oxygenates can increase the undesirable smog-forming and
toxic emissions. The major reduction in the reactivity of exhaust and
evaporative emissions will occur with reformulated gasolines, due to
be introduced in January 1995, which have oxygenates and major
composition changes to the hydrocarbon component. Oxygenates do not
necessarily reduce all individual exhaust toxins, nor are they
intended to.
Oxygenates have significantly different physical properties to
hydrocarbons, and the levels that can be added to gasolines are
controlled by the EPA in the US, with waivers being granted for some
combinations. The change to reformulated gasoline requires oxygenates,
but also that the hydrocarbon composition must be significantly more
modified than the existing oxygenated gasolines to reduce unsaturates,
volatility, benzene, and the reactivity of emissions.
Oxygenates that are added to gasoline function in two ways. Firstly
they have high blending octane, and so can replace high octane
aromatics in the fuel. These aromatics are responsible for
disproportionate amounts of CO and HC exhaust emissions. This is
called the "aromatic substitution effect". Oxygenates also cause
engines without sophisticated engine management systems to move to the
lean side of stoichiometry, thus reducing emissions of CO ( 2% oxygen
can reduce CO by 16% ) and HC ( 2% oxygen can reduce HC by 10%).
However, on vehicles with engine management systems, the fuel volume
will be increased to bring the stoichiometry back to the preferred
optimum setting. Oxygen in the fuel can not contribute energy,
consequently the fuel has less energy content. For the same efficiency
and power output, more fuel has to be burnt, and the slight
improvements in efficiency that oxygenates provide on some engines
usually do not completely compensate for the oxygen [12].
There are huge number of chemical mechanisms involved in the pre-flame
reactions of gasoline combustion. Although both alkyl leads and
oxygenates are effective at suppressing knock, the chemical modes
through which they act are entirely different. MTBE works by retarding
the progress of the low temperature or cool-flame reactions, consuming
radical species, particularly OH radicals and producing isobutene. The
isobutene in turn consumes additional OH radicals and produces
unreactive, resonantly stabilised radicals such as allyl and methyl
allyl, as well as stable species such as allene, which resist further
oxidation [13,14].
4.6 Why were alkyl lead compounds added?
The efficiency of a spark-ignited gasoline engine can be related to
the compression ratio up to at least compression ratio 17:1 [15].
However any "knock" caused by the fuel will rapidly mechanically
destroy an engine, and General Motors was having major problems trying
to improve engines without inducing knock. The problem was to identify
economic additives that could be added to gasoline or kerosine to
prevent knock, as it was apparent that engine development was being
hindered. The kerosine for home fuels soon became a secondary issue,
as the magnitude of the automotive knock problem increased throughout
the 1910s, and so more resources were poured into the quest for an
effective "anti-knock". A higher octane aviation gasoline was required
urgently once the US entered WWI, and almost every possible chemical (
including melted butter ) was tested for anti-knock ability [16].
Originally, iodine was the best anti-knock available, but was not a
practical gasoline additive, and was used as the benchmark. In 1919
aniline was found to have superior antiknock ability to iodine, but
also was not a practical additive, however aniline became the
benchmark anti-knock, and various compounds were compared to it. The
discovery of tetra ethyl lead, and the scavengers required to remove
it from the engine were made by teams lead by Thomas Midgley Jr. in
1922 [7,8,16]. They tried selenium oxychloride which was an excellent
antiknock, however it reacted with iron and "dissolved" the engine.
Midgley was able to predict that other organometallics would work, and
slowly focused on organoleads. They then had to remove the lead, which
would otherwise accumulate and coat the engine and exhaust system with
lead. They discovered and developed the halogenated lead scavengers
that are still used in leaded fuels. The scavengers, ( ethylene
dibromide and ethylene dichloride ), function by providing halogen
atoms that react with the lead to form volatile lead halide salts that
can escape out the exhaust. The quantity of scavengers added to the
alkyl lead concentrate is calculated according to the amount of lead
present. If sufficient scavenger is added to theoretically react with
all the lead present, the amount is called one "theory". Typically,
1.0 to 1.5 theories are used, but aviation gasolines must only use one
theory. This ensures there is no excess bromine that could react with
the engine. The alkyl leads rapidly became the most cost-effective
method of enhancing octane.
The development of the alkyl leads ( tetra methyl lead, tetra ethyl
lead ) and the toxic halogenated scavengers meant that petroleum
refiners could then configure refineries to produce hydrocarbon
streams that would increase octane with small quantities of alkyl
lead. If you keep adding alkyl lead compounds, the lead response of
the gasoline decreases, and so there are economic limits to how much
lead should be added.
Up until the late 1960s, alkyl leads were added to gasolines in
increasing concentrations to obtain octane. The limit was 1.14g Pb/l,
which is well above the diminishing returns part of the lead response
curve for most refinery streams, thus it is unlikely that much fuel
was ever made at that level. I believe 1.05 was about the maximum, and
articles suggest that 1970 100 RON premiums were about 0.7-0.8 g Pb/l
and 94 RON regulars 0.6-0.7 g Pb/l, which matches published lead
response data [17] eg.
For Catalytic Reformate Straight Run Naphtha.
Lead g/l Research Octane Number
0 96 72
0.1 98 79
0.2 99 83
0.3 100 85
0.4 101 87
0.5 101.5 88
0.6 102 89
0.7 102.5 89.5
0.8 102.75 90
The alkyl lead anti-knocks work in a different stage of the
pre-combustion reaction to oxygenates. In contrast to oxygenates, the
alkyl lead interferes with hydrocarbon chain branching in the
intermediate temperature range where HO2 is the most important radical
species. Lead oxide, either as solid particles, or in the gas phase,
reacts with HO2 and removes it from the available radical pool,
thereby deactivating the major chain branching reaction sequence that
results in undesirable, easily-autoignitable hydrocarbons [13,14].
4.7 Why not use other organometallic compounds?
As the toxicity of the alkyl lead and the halogenated scavengers
became of concern, alternatives were considered. The most famous of
these is methylcyclopentadienyl manganese tricarbonyl (MMT), which was
used in the USA until banned by the EPA from 27 Oct 1978 [18], but is
approved for use in Canada and Australia. It is more expensive than
alkyl leads and has been reported to increase unburned hydrocarbon
emissions and block exhaust catalysts [19]. Other compounds that
enhance octane have been suggested, but usually have significant
problems such as toxicity, cost, increased engine wear etc.. Examples
include dicyclopentadienyl iron and nickel carbonyl.
4.8 What do the refining processes do?
Crude oil contains a wide range of hydrocarbons, organometallics and
other compounds containing sulfur, nitrogen etc. The HCs contain
between 1 and 60 carbon atoms. Gasoline requires hydrocarbons with
carbon atoms between 3 and 12, arranged in specific ways to provide
the desirable properties. Obviously, a refinery has to either sell the
remainder as marketable products, or convert the larger molecules into
smaller gasoline molecules.
A refinery will distill crude oil into various fractions and,
depending on the desired final products, will further process and
blend those fractions. Typical final products could be:- gases for
chemical synthesis and fuel (CNG), liquified gases (LPG), butane,
aviation and automotive gasolines, aviation and lighting kerosines,
diesels, distillate and residual fuel oils, lubricating oil base
grades, paraffin oils and waxes. Many of the common processes are
intended to increase the yield of blending feedstocks for gasolines.
Typical modern refinery processes for gasoline components include
- Catalytic cracking - breaks larger, higher-boiling, hydrocarbons
into gasoline range product that contains 30% aromatics and 20-30%
olefins.
- Hydrocracking - cracks and adds hydrogen to molecules, producing a
more saturated, stable, gasoline fraction.
- Isomerisation - raises gasoline fraction octane by converting
straight chain hydrocarbons into branched isomers.
- Reforming - converts saturated, low octane, hydrocarbons into
higher octane product containing about 60% aromatics.
- Alkylation - reacts gaseous olefin streams with isobutane to
produce liquid high octane iso-alkanes.
The changes that the Clean Air Act and other legislation ensures that
the refineries will continue to modify their processes to produce a
less volatile gasoline with fewer toxins and toxic emissions. Options
include:-
- Reducing the "severity" of reforming to reduce aromatic
production.
- Distilling the C5/C6 fraction from reformer feeds and treating
that stream to produce non-aromatic high octane components.
- Distilling the higher boiling fraction ( which contains 80-100% of
aromatics that can be hydrocracked ) from catalytic cracker
product [20].
- Convert butane to isobutane or isobutylene for alkylation or MTBE
feed.
4.9 What energy is released when gasoline is burned?
It is important to note that the theoretical energy content of
gasoline when burned in air is only related to the hydrogen and carbon
contents. Octane rating is not fundamentally related to the energy
content, and the actual hydrocarbon and oxygenate components used in
the gasoline will determine both the energy release and the anti-knock
rating.
Two important reactions are:-
- C + O2 = CO2
- H + O2 = H2O
The mass or volume of air required to provide sufficient oxygen to
achieve this complete combustion is the "stoichiometric" mass or
volume of air. Insufficient air = "rich", and excess air = "lean", and
the stoichiometric mass of air is related to the carbon:hydrogen ratio
of the fuel. The procedures for calculation of stoichiometric air/fuel
ratios are fully documented in an SAE standard [21].
Atomic masses used are:- Hydrogen = 1.00794, Carbon = 12.011, Oxygen =
15.994, Nitrogen = 14.0067, and Sulfur = 32.066.
The composition of sea level air ( 1976 data, hence low CO2 value ) is
Gas Fractional Molecular Weight Relative
Species Volume kg/mole Mass
N2 0.78084 28.0134 21.873983
O2 0.209476 31.9988 6.702981
Ar 0.00934 39.948 0.373114
CO2 0.000314 44.0098 0.013919
Ne 0.00001818 20.179 0.000365
He 0.00000524 4.002602 0.000021
Kr 0.00000114 83.80 0.000092
Xe 0.000000087 131.29 0.000011
CH4 0.000002 16.04276 0.000032
H2 0.0000005 2.01588 0.000001
---------
Air 28.964419
For normal heptane C7H16 with a molecular weight = 100.204
C7H16 + 11O2 = 7CO2 + 8H2O
thus 1.000 kg of C7H16 required 3.513 kg of O2 = 15.179 kg air.
The chemical stoichiometric combustion of hydrocarbons with oxygen can
be written as:-
CxHy + (x + (y/4))O2 -> xCO2 + (y/2)H2O
Often, for simplicity, the remainder of air is assumed to be nitrogen,
which can be added to the equation when exhaust compositions are
required. As a general rule, maximum power is achieved at slightly
rich, whereas maximum fuel economy is achieved at slightly lean.
The energy content of the gasoline is obtained by burning all the fuel
inside a bomb calorimeter and measuring the temperature increase. The
energy available depends on what happens to the water produced from
the combustion of the hydrogen. If the water remains as a gas, then it
cannot release the heat of vaporisation, thus producing the Nett
Calorific Value. If the water were condensed back to the original fuel
temperature, then Gross Calorific Value of the fuel, which will be
larger, is obtained.
The calorific values are fairly constant for families of HCs, which is
not surprising, given their fairly consistent carbon/hydrogen ratios.
For liquid ( l ) or gaseous ( g ) fuel converted to gaseous products -
except for the 2-methylbutene-2, where only gaseous is reported. * =
Blending Octane Number
Typical Heats of Combustion are [22]:-
Fuel State Heat of Combustion Research Motor
MJ/kg Octane Octane
n-heptane l 44.592 0 0
g 44.955
i-octane l 44.374 100 100
g 44.682
toluene l 40.554 124* 112*
g 40.967
2-methylbutene-2 44.720 176* 141*
Because all the data is available, the calorific value of fuels can be
estimated quite accurately from hydrocarbon fuel properties such as
the density, sulfur content, and aniline point ( which indicates the
aromatics content ).
It should be noted that because oxygenates contain oxygen that can not
provide energy, they will have significantly lower energy contents.
They are added to provide octane, not energy. For an engine that can
be optimised for oxygenates, more fuel is required to obtain the same
power, but they can burn slightly more efficiently, thus the power
ratio is not identical to the energy content ratio. They also require
more energy to vaporise.
Energy Content Heat of Vaporisation Oxygen Content
Nett MJ/kg MJ/kg wt%
Methanol 19.95 1.154 49.9
Ethanol 26.68 0.913 34.7
MTBE 35.18 0.322 18.2
ETBE 36.29 0.310 15.7
TAME 36.28 0.323 15.7
Gasoline 42 - 44 0.297 0.0
Typical values for commercial fuels in megajoules/kilogram are [23]:-
Gross Nett
Hydrogen 141.9 120.0
Carbon to Carbon monoxide 10.2 -
Carbon to Carbon dioxide 32.8 -
Sulfur to sulfur dioxide 9.16 -
Natural Gas 53.1 48.0
Liquified petroleum gas 49.8 46.1
Aviation gasoline 46.0 44.0
Automotive gasoline 45.8 43.8
Kerosine 46.3 43.3
Diesel 45.3 42.5
Obviously, for automobiles, the nett calorific value is appropriate.
The calorific value is the maximum energy that can be obtained from
the fuel, but the reality of modern SI engines is that efficiencies of
20-40% may be obtained, this limit being due to engineering and
material constraints that prevent optimum combustion conditions being
used. The CI engine can achieve higher efficiencies, usually over a
wider operating range as well.
4.10 What are the gasoline specifications?
Gasolines are usually defined by government regulation, where
properties and test methods are clearly defined. In the US, several
government and state bodies can specify gasoline properties. The US
gasoline specifications and test methods are listed in several readily
available publications, including the Society of Automotive Engineers
(SAE) [24], and the American Society for Testing Materials (ASTM)
[25]. The 1994 ASTM edition has:-
D4814-93a Specification for Automotive Spark-Ignition Engine Fuel.
This specification lists various properties that all fuels have to
comply with, and may be updated throughout the year. Typical
properties are:-
4.10.1 Vapour Pressure and Distillation Classes.
6 different classes according to location and/or season.
As gasoline is distilled, the temperatures at which various fractions
are evaporated are calculated. Specifications define the temperatures
at which various percentages of the fuel are evaporated. Distillation
limits include maximum temperatures that 10% is evaporated (50-70C),
50% is evaporated (110-121C), 90% is evaporated (185-190C), and the
final boiling point (225C). A minimum temperature for 50% evaporated
(77C), and a maximum amount of Residue (2%) after distillation. Vapour
pressure limits for each class ( 54, 62, 69, 79, 93, 103 kPa ) are
also specified. Note that the EPA has issued a waiver that does not
require gasoline/ethanol blends to meet the required specifications.
4.10.2 Vapour Lock Protection Classes
5 classes for vapour lock protection, according to location and/or
season. The limit is a maximum Vapour/Liquid ratio of 20 at test
temperatures of 41, 47, 51, 56, 60C.
4.10.3 Antiknock Index ( aka (RON+MON)/2, "Pump Octane" )
The ( Research Octane Number + Motor Octane Number ) divided by two.
Limits are not specified, but changes in engine requirements according
season and location are discussed. Fuels with an Antiknock index of
87, 89, 91 ( Unleaded), and 88 ( Leaded ) are listed as typical for
the US.
4.10.4 Lead Content
Leaded = 1.1 g Pb / L maximum, and Unleaded = 0.013 g Pb / L maximum.
4.10.5 Copper strip corrosion
Ability to tarnish clean copper, indicating the presence of any
corrosive sulfur compounds
4.10.6 Maximum Sulfur content
Sulfur adversely affects exhaust catalysts and fuel hydrocarbon lead
response, and also may be emitted as polluting sulfur oxides.
Leaded = 0.15 %mass maximum, and Unleaded = 0.10 %mass maximum.
Typical US gasoline levels are 0.03 %mass.
4.10.7 Maximum Existent Gum
Limits the amount of gums present in fuel at the time of testing to 5
mg/100mls. The results do not correlate well with actual engine
deposits caused by fuel vaporisation [26].
4.10.8 Minimum Oxidation Stability
This ensures the fuel remains chemically stable, and does not form
additional gums during periods in distribution systems, which can be
up to 3-6 months. The sample is heated with oxygen inside a pressure
vessel, and the delay until significant oxygen uptake is measured.
4.10.9 Water Tolerance
Highest temperature that causes phase separation of oxygenated fuels.
The limits vary according to location and month. For Alaska - North of
62 latitude, it changes from -41C in Dec/Jan to 9C in July, but
remains 10C all year in Hawaii.
As well as the above, there are various restrictions introduced by the
Clean Air Act and state bodies such as California's Air Resources
Board (CARB) that often have more stringent limits for the above
properties, as well as additional limits. The Clean Air Act also
specifies some regions that exceed air quality standards have to use
reformulated gasolines (RFGs) all year, starting January 1995. Other
regions are required to use oxygenated gasolines for four winter
months, beginning November 1992. The RFGs also contain oxygenates.
Metropolitan regions with severe ozone air quality problems must use
reformulated gasolines in 1995 that;- contain at least 2.0 wt% oxygen,
reduce 1990 volatile organic carbon compounds by 15%, and reduce
specified toxic emissions by 15% (1995) and 25% (2000). Metropolitan
regions that exceeded carbon monoxide limits were required to use
gasolines with 2.7 wt% oxygen during winter months, starting in 1992.
Because phosphorus adversely affects exhaust catalysts, the EPA limits
phosphorus in all gasolines to 0.0013 gP/L.
The 1990 Clean Air Act (CAA) amendments and CARB phase 2 (1996)
specifications for reformulated gasoline establish the following
limits, compared with typical 1990 gasoline. Because of a lack of
data, the EPA were unable to define the CAA required parameters , so
they instituted a two-stage system. The first stage, the "Simple
Model" is an interim stage that run from 1/Jan/1995 to 1/May/1997. The
second stage, the "Complex Model" would be developed, with the
following parameters likely to be controlled - reid vapour pressure,
benzene, oxygen, sulfur, olefins distillation ( 90% Evaporated ), and
aromatics. Each refiner must have their RFG recertified using the
Complex model by 1/May/1997 [27].
1990 Clean Air Act CARB
benzene 2 % 1 % maximum 1.0 vol% maximum
oxygen 0.2 % 2 % minimum 1.8-2.0 mass%
sulfur 150 ppm no increase 40 ppm
aromatics 32.0 % 25 % maximum 25 vol% maximum
olefins 9.9 % 5 % maximum 6 vol% maximum
reid vapour pressure 60 kPa 56 kPa (north) 48 kPa
50 kPa (south)
90% evaporated 170 C - 149 C
These regulations also specify emissions criteria. eg CAA specifies no
increase in nitric oxides (NOx) emissions, reductions in VOC by 15%
during the ozone season, and specified toxins by 15% all year. These
criteria indirectly establish vapour pressure and composition limits
that refiners have to meet. Note that the EPA also can issue CAA
Section 211 waivers that allow refiners to choose which oxygenates
they use. In 1981, the EPA also decided that fuels with up to 2%
alcohols and ethers (except methanol) were "substantially similar" to
1974 unleaded gasoline, and thus were not "new" gasoline additives.
That level was increased to 2.7 wt% in 1991. Some other oxygenates
have also been granted waivers, eg ethanol to 3.5 wt% in 1979/1982,
and tert-butyl alcohol to 3.5 wt% in 1981.
4.11 What are the effects of the specified fuel properties?
Volatility
This affects evaporative emissions and driveability, it is the
property that must change with location and season. Fuel for
mid-summer Arizona would be difficult to use in mid-winter Alaska. The
US is divided into zones, according to altitude and seasonal
temperatures, and the fuel volatility is adjusted accordingly.
Incorrect fuel may result in difficult starting in cold weather,
carburetter icing, vapour lock in hot weather, and crankcase oil
dilution. Volatility is controlled by distillation and vapour pressure
specifications. The higher boiling fractions of the gasoline have
significant effects on the emission levels of undesirable hydrocarbons
and aldehydes, and a reduction of 40C in the final boiling point will
reduce the levels of benzene, butadiene, formaldehyde and acetaldehyde
by 25%, and will reduce HC emissions by 20% [28].
Combustion Characteristics
As gasolines contain mainly hydrocarbons, the only significant
variable between different grades is the octane rating of the fuel, as
most other properties are similar. Octane is discussed in detail in
Section 6. There are only slight differences in combustion
temperatures ( most are around 2000C in isobaric adiabatic combustion
[29]). Note that the actual temperature in the combustion chamber is
also determined by other factors, such as load and engine design. The
addition of oxygenates changes the pre-flame reaction pathways, and
also reduces the energy content of the fuel. The levels of oxygen in
the fuel is regulated according to regional air quality standards.
Stability
Motor gasolines may be stored up to six months, consequently they must
not form gums which may precipitate. Gums are usually the result of
copper-catalysed reactions of the unsaturated HCs, so antioxidants and
metal deactivators are added. Existent Gum is used to measure the gum
in the fuel at the time tested, whereas the Oxidation Stability
measures the time it takes for the gasoline to break down at 100C with
100psi of oxygen. A 240 minutes test period has been found to be
sufficient for most storage and distribution systems.
Corrosiveness
Sulfur in the fuel creates corrosion, and when combusted will form
corrosive gases that attack the engine, exhaust and environment.
Sulfur also adversely affects the alkyl lead octane response and may
poison exhaust catalysts. The copper strip corrosion test and the
sulfur specification are used to ensure fuel quality. The copper strip
test measures active sulfur, whereas the sulfur content reports the
total sulfur present.
4.12 Are brands different?
Yes. The above specifications are intended to ensure minimal quality
standards are maintained, however as well as the fuel hydrocarbons,
the manufacturers add their own special ingredients to provide
additional benefits. A quality gasoline additive package would
include:-
- octane-enhancing additives ( improve octane ratings )
- anti-oxidants ( inhibit gum formation, improve stability )
- metal deactivators ( inhibit gum formation, improve stability )
- deposit modifiers ( reduce deposits, spark-plug fouling and
preignition )
- surfactants ( prevent icing, improve vaporisation, inhibit
deposits, reduce NOx emissions )
- freezing point depressants ( prevent icing )
- corrosion inhibitors ( prevent gasoline corroding storage tanks )
- dyes ( product colour for safety or regulatory purposes ).
During the 1980s significant problems with deposits accumulating on
intake valve surfaces occurred as new fuel injections systems were
introduced. These intake valve deposits (IVD) were different to the
injector deposits, in part because the valve can reach 300C. Engine
design changes that prevent deposits usually consist of ensuring the
valve is flushed with liquid gasoline, and provision of adequate valve
rotation. Gasoline factors that cause deposits are the presence of
alcohols or olefins. Gasoline manufacturers now routinely use
additives that prevent IVD and also maintain the cleanliness of
injectors. These usually include a surfactant and light oil to
maintain the wetting of important surfaces. A more detailed
description of additives is provided in Section 9.1.
Texaco demonstrated that a well-formulated package could improve fuel
economy, reduce NOx emissions, and restore engine performance because,
as well as the traditional liquid-phase deposit removal, some
additives can work in the vapour phase to remove existing engine
deposits without adversely affecting performance ( as happens when
water is poured into a running engine to remove carbon deposits:-)
)[30]. Most suppliers of quality gasolines will formulate similar
additives into their products, and cheaper lines are less like to have
such additives added. As different brands use different additives and
oxygenates, it is probable that important parameters, such as octane
distribution, are different, even though the pump octane ratings are
the same.
So, if you know your car is well-tuned, and in good condition, but the
driveability is pathetic on the correct octane, try another brand.
Remember that the composition will change with the season, so if you
lose driveability, try yet another brand. As various Clean Air Act
changes are introduced over the next few years, gasoline will continue
to change.
4.13 What is a typical composition?
There seems to be a perception that all gasolines of one octane grade
are chemically similar, and thus general rules can be promulgated
about "energy content ", "flame speed", "combustion temperature" etc.
etc.. Nothing is further from the truth. The behaviour of manufactured
gasolines in octane rating engines can be predicted, using previous
octane ratings of special blends intended to determine how a
particular refinery stream responds to an octane-enhancing additive.
Refiners can design and reconfigure refineries to efficiently produce
a wide range of gasolines feedstocks, depending on market and
regulatory requirements.
The last 10 years of various compositional changes to gasolines for
environmental and health reasons have resulted in fuels that do not
follow historical rules, and the regulations mapped out for the next
decade also ensure the composition will remain in a state of flux. The
reformulated gasoline specifications, especially the 1/May/1997
Complex model, will probably introduce major reductions in the
distillation range, as well as the various limits on composition and
emissions.
I'm not going to list all 500+ HCs in gasolines, but the following are
representative of the various classes typically present in a gasoline.
The numbers after each chemical are:- Research Blending Octane : Motor
Blending Octane : Boiling Point (C): Density (g/ml _at_ 15C) : Minimum
Autoignition Temperature (C). It is important to realise that the
Blending Octanes are derived from a 20% mix of the HC with a 60:40
iC8:nC7 base, and the extrapolation of this 20% to 100%. This is
different from rating the pure fuel, which often requires adjustment
of the test engine conditions outside the acceptable limits of the
rating methods. Generally the actual octanes of the pure fuel are
similar for the alkanes, but are up to 30 octane numbers lower than
the blending octanes for the aromatics and olefins [31].
A traditional composition I have dreamed up would be like the
following, whereas newer oxygenated fuels reduce the aromatics and
olefins, narrow the boiling range, and add oxygenates up to about
12-15% to provide the octane.
15% n-paraffins RON MON BP d AIT
n-butane 113 : 114 : -0.5: gas : 370
n-pentane 62 : 66 : 35 : 0.626 : 260
n-hexane 19 : 22 : 69 : 0.659 : 225
n-heptane (0:0 by definition) 0 : 0 : 98 : 0.684 : 225
n-octane -18 : -16 : 126 : 0.703 : 220
( you would not want to have the following alkanes in gasoline,
so you would never blend kerosine with gasoline )
n-decane -41 : -38 : 174 : 0.730 : 210
n-dodecane -88 : -90 : 216 : 0.750 : 204
n-tetradecane -90 : -99 : 253 : 0.763 : 200
30% iso-paraffins
2-methylpropane 122 : 120 : -12 : gas : 460
2-methylbutane 100 : 104 : 28 : 0.620 : 420
2-methylpentane 82 : 78 : 62 : 0.653 : 306
3-methylpentane 86 : 80 : 64 : 0.664 : -
2-methylhexane 40 : 42 : 90 : 0.679 :
3-methylhexane 56 : 57 : 91 : 0.687 :
2,2-dimethylpentane 89 : 93 : 79 : 0.674 :
2,2,3-trimethylbutane 112 : 112 : 81 : 0.690 : 420
2,2,4-trimethylpentane 100 : 100 : 98 : 0.692 : 415
( 100:100 by definition )
12% cycloparaffins
cyclopentane 141 : 141 : 50 : 0.751 : 380
methylcyclopentane 107 : 99 : 72 : 0.749 :
cyclohexane 110 : 97 : 81 : 0.779 : 245
methylcyclohexane 104 : 84 : 101 : 0.770 : 250
35% aromatics
benzene 98 : 91 : 80 : 0.874 : 560
toluene 124 : 112 : 111 : 0.867 : 480
ethyl benzene 124 : 107 : 136 : 0.867 : 430
meta-xylene 162 : 124 : 138 : 0.868 : 463
para-xylene 155 : 126 : 138 : 0.866 : 530
ortho-xylene 126 : 102 : 144 : 0.870 : 530
3-ethyltoluene 162 : 138 : 158 : 0.865 :
1,3,5-trimethylbenzene 170 : 136 : 163 : 0.864 :
1,2,4-trimethylbenzene 148 : 124 : 168 : 0.889 :
8% olefins
2-pentene 154 : 138 : 37 : 0.649 :
2-methylbutene-2 176 : 140 : 36 : 0.662 :
2-methylpentene-2 159 : 148 : 67 : 0.690 :
cyclopentene 171 : 126 : 44 : 0.774 :
( the following olefins are not present in significant amounts
in gasoline, but have some of the highest blending octanes )
1-methylcyclopentene 184 : 146 : 75 : 0.780 :
1,3 cyclopentadiene 218 : 149 : 42 : 0.805 :
dicyclopentadiene 229 : 167 : 170 : 1.071 :
Oxygenates
Published octane values vary a lot because the rating conditions are
significantly different to standard conditions, for example the API
Project 45 numbers used above for the hydrocarbons, reported in 1957,
gave MTBE blending RON as 148 and MON as 146, however that was based
on the lead response, whereas today we use MTBE in place of lead.
methanol 133 : 105 : 65 : 0.796 : 385
ethanol 129 : 102 : 78 : 0.794 : 365
iso propyl alcohol 118 : 98 : 82 : 0.790 : 399
methyl tertiary butyl ether 116 : 103 : 55 : 0.745 :
ethyl tertiary butyl ether 118 : 102 : 72 : 0.745 :
tertiary amyl methyl ether 111 : 98 : 86 : 0.776 :
There are some other properties of oxygenates that have to be
considered when they are going to be used as fuels, particularly their
ability to form very volatile azeotropes that cause the fuel's vapour
pressure to increase, the chemical nature of the emissions, and their
tendency to separate into a separate water/oxygenate phase when water
is present. The reformulated gasolines address these problems more
successfully than the original oxygenated gasolines.
Before you rush out to make a highly aromatic or olefinic gasoline to
produce a high octane fuel, remember they have other adverse
properties, eg the aromatics attack elastomers and generate smoke, and
the olefins are unstable ( besides smelling foul ) and form gums. The
art of correctly formulating a gasoline that does not cause engines to
knock apart, does not cause vapour lock in summer - but is easy to
start in winter, does not form gums and deposits, burns cleanly
without soot/residues, and does not dissolve or poison the car
catalyst or owner, is based on knowledge of the gasoline composition.
4.14 Is gasoline toxic or carcinogenic?
There are several known toxins in gasoline, some of which are
confirmed human carcinogens. The most famous of these toxins are lead
and benzene, and both are regulated. The other aromatics and some
toxic olefins are also controlled. Lead alkyls also require ethylene
dibromide and/or ethylene dichloride scavengers to be added to the
gasoline, both of which are suspected human carcinogens. In 1993 an
International Symposium on the Health Effects of Gasoline was held
[32]. Major review papers on the carcinogenic, neurotoxic,
reproductive and developmental toxicity of gasoline, additives, and
oxygenates were presented. The oxygenates are also being evaluated for
carcinogenicity, and even ethanol and ETBE may be carcinogens. It
should be noted that the oxygenated gasolines were not expected to
reduce the toxicity of the emissions, however the reformulated
gasolines will produce different emissions, and specific toxins must
be reduced by 15% all year.
There is little doubt that gasoline is full of toxic chemicals, and
should therefore be treated with respect. However the biggest danger
remains the flammability, and the relative hazards should always be
kept in perspective. The major toxic risk from gasolines comes from
breathing the tailpipe, evaporative, and refuelling emissions, rather
than occasional skin contact from spills. Breathing vapours and skin
contact should always be minimised.
4.15 Is unleaded gasoline more toxic than leaded?
The short answer is no. However that answer is not global, as some
countries have replaced the lead compound octane-improvers with
aromatic or olefin octane-improvers without introducing exhaust
catalysts. Some aromatics are more toxic that paraffins.
Unfortunately, the manufacturers of alkyl lead compounds have embarked
on a worldwide misinformation campaign in countries considering
emulating the lead-free US. The use of lead precludes the use of
exhaust catalysts, thus the emissions of aromatics are only slightly
diminished, and other pollutants can not reduced by exhaust catalysts.
The use of unleaded on modern vehicles with engine management systems
and catalysts can reduce aromatic emissions to 10% of the level of
vehicles without catalysts [33]. Alkyl lead additives can only
substitute for some of the aromatics in gasoline, consequently they do
not eliminate aromatics, which will produce benzene emissions [34].
Alkyl lead additives also require toxic organohalogen scavengers,
which also react in the engine to form and emit other organohalogens,
including highly toxic dioxin [35]. Leaded fuels emit lead,
organohalogens, and much higher levels of regulated toxins because
they preclude the use of exhaust catalysts. In the USA the gasoline
composition is being changed to reduce fuel toxins ( olefins,
aromatics ) as well as emissions of specific toxins.
Subject: 5. Why is Gasoline Composition Changing?
5.1 Why pick on cars and gasoline?
Cars emit several pollutants as combustion products out the tailpipe,
(tailpipe emissions), and as losses due to evaporation (evaporative
emissions, refuelling emissions). The volatile organic carbon (VOC)
emissions from these sources, along with nitrogen oxides (NOx)
emissions from the tailpipe, will react in the presence of ultraviolet
light (wavelengths of less than 430nm) to form ground-level
(tropospheric) ozone, which is one of the major components of
photochemical smog [36]. Smog has been a major pollution problem ever
since coal-fired power stations were developed in urban areas, but
their emissions are being cleaned up. Now it's the turn of the
automobile.
Cars currently use gasoline that is derived from fossil fuels, thus
when gasoline is burned to completion, it produces additional CO2 that
is added to the atmospheric burden. The effect of the additional CO2
on the global environment is not known, but the quantity of man-made
emissions of fossil fuels must cause the system to move to a new
equilibrium. Even if current research doubles the efficiency of the IC
engine/gasoline combination, and reduces HC, CO, NOx, SOx, VOCs,
particulates, and carbonyls, the amount of carbon dioxide from the use
of fossil fuels may still cause global warming. More and more
scientific evidence is accumulating that warming is occurring [37].
The issue is whether it is natural, or induced by human activities.
There are international agreements to limit CO2 emissions to 1990
levels, a target that will require more efficient, lighter, or
appropriately-sized vehicles, - if we are to maintain the current
usage. One option is to use "renewable" fuels in place of fossil
fuels. Consider the amount of energy-related CO2 emissions for
selected countries in 1990 [38].
CO2 Emissions
( tonnes/year/person )
USA 20.0
Canada 16.4
Australia 15.9
Germany 10.4
United Kingdom 8.6
Japan 7.7
New Zealand 7.6
The number of new vehicles provides an indication of the magnitude of
the problem. Although vehicle engines are becoming more efficient, the
distance travelled is increasing, resulting in a gradual increase of
gasoline consumption. The world production of vehicles (in thousands)
over the last few years was [39];-
Cars
Region 1990 1991 1992 1993
Africa 222 213 194 201
Asia-Pacific 12,064 12,112 11,869 11,467
Central & South America 800 888 1,158 1,524
Eastern Europe 2,466 984 1,726 1,783
Middle East 35 24 300 377
North America 7,762 7,230 7,470 8,172
Western Europe 13,688 13,286 13,097 11,124
Total World 37,039 34,739 35,815 34,649
Trucks ( including heavy trucks and buses )
Region 1990 1991 1992 1993
Africa 133 123 108 109
Asia-Pacific 5,101 5,074 5,117 5,054
Central & South America 312 327 351 417
Eastern Europe 980 776 710 708
Middle East 36 28 100 110
North America 4,851 4,554 5,371 6,037
Western Europe 1,924 1,818 1,869 1,345
Total World 13,336 12,701 13,627 13,779
To fuel all operating vehicles, considerable quantities of gasoline
and diesel have to be consumed. Major consumption in 1993 of gasoline
and middle distillates ( which may include some heating fuels, but not
fuel oils ) in million tonnes.
Gasoline Middle Distillates
USA 335.6 233.9
Canada 25.0 24.4
Western Europe 166.0 264.0
Japan 56.4 89.6
Total World 802.0 989.0
The USA consumption of gasoline increased from 294.4 (1982) to 335.6
(1989) then dipped to 324.2 (1991), and has continued to rise since
then to reach 335.6 million tonnes in 1993. In 1993 the total world
production of crude oil was 3164.8 million tonnes, of which the USA
consumed 787.5 million tonnes [40]. Transport is a very significant
user of crude oil products, thus improving the efficiency of
utilisation, and minimising pollution from vehicles, can produce
immediate reductions in emissions of CO2, HCs, VOCs, CO, NOx,
carbonyls, and other chemicals.
5.2 Why are there seasonal changes?
Only gaseous hydrocarbons burn, consequently if the air is cold, then
the fuel has to be very volatile. But when summer comes, a volatile
fuel can boil and cause vapour lock, as well as producing high levels
of evaporative emissions. The solution was to adjust the volatility of
the fuel according to altitude and ambient temperature. This
volatility change has been automatically performed for decades by the
oil companies without informing the public of the changes. It is one
reason why storage of gasoline through seasons is not a good idea.
Gasoline volatility is being reduced as modern engines, with their
fuel injection and management systems, can automatically compensate
for some of the changes in ambient conditions - such as altitude and
air temperature, resulting in acceptable driveability using less
volatile fuel.
5.3 Why were alkyl lead compounds removed?
" With the exception of one premium gasoline marketed on the east
coast and southern areas of the US, all automotive gasolines from the
mid-1920s until 1970 contained lead antiknock compounds to increase
antiknock quality. Because lead antiknock compounds were found to be
detrimental to the performance of catalytic emission control system
then under development, U.S. passenger car manufacturers in 1971 began
to build engines designed to operate satisfactorily on gasolines of
nominal 91 Research Octane Number. Some of these engines were designed
to operate on unleaded fuel while others required leaded fuel or the
occasional use of leaded fuel. The 91 RON was chosen in the belief
that unleaded gasoline at this level could be made available in
quantities required using then current refinery processing equipment.
Accordingly, unleaded and low-lead gasolines were introduced during
1970 to supplement the conventional gasolines already available.
Beginning with the 1975 model year, most new car models were equipped
with catalytic exhaust treatment devices as one means of compliance
with the 1975 legal restrictions in the U.S. on automobile emissions.
The need for gasolines that would not adversely affect such catalytic
devices has led to the large scale availability and growing use of
unleaded gasolines, with all late-model cars requiring unleaded
gasoline."[41].
There was a further reason why alkyl lead compounds were subsequently
reduced, and that was the growing recognition of the highly toxic
nature of the emissions from a leaded-gasoline fuelled engine. Not
only were toxic lead emissions produced, but the added toxic lead
scavengers ( ethylene dibromide and ethylene dichloride ) could react
with hydrocarbons to produce highly toxic organohalogen emissions such
as dioxin. Even if catalysts were removed, or lead-tolerant catalysts
discovered, alkyl lead compounds would remain banned because of their
toxicity and toxic emissions [42].
5.4 Why are evaporative emissions a problem?
As tailpipe emissions are reduced due to improved exhaust emission
control systems, the hydrocarbons produced by evaporation of the
gasoline during distribution, vehicle refuelling, and from the
vehicle, become more and more significant. A recent European study
found that 40% of man-made volatile organic compounds came from
vehicles [43]. Many of the problem hydrocarbons are the aromatics and
olefins that have relatively high octane values. Any sensible strategy
to reduce smog and toxic emissions will attack evaporative and
tailpipe emissions.
The health risks to service station workers, who are continuously
exposed to refuelling emissions remain a concern [44]. Vehicles will
soon be required to trap the refuelling emissions in larger carbon
canisters, as well as the normal evaporative emissions that they
already capture. This recent decision went in favour of the oil
companies, who were opposed by the auto companies. The automobile
manufacturers felt the service station should trap the emissions. The
activated carbon canisters adsorb organic vapours, and these are
subsequently desorbed from the canister and burnt in the engine during
normal operation, once certain vehicle speeds and coolant temperatures
are reached. A few activated carbons used in older vehicles do not
function efficiently with oxygenates.
5.5 Why control tailpipe emissions?
Tailpipe emissions were responsible for the majority of pollutants in
the late 1960s after the crankcase emissions had been controlled.
Ozone levels in the Los Angeles basin reached 450-500ppb in the early
1970s, well above the typical background of 30-50ppb [45].
Tuning a carburetted engine can only have a marginal effect on
pollutant levels, and there still had to be some frequent, but
long-term, assessment of the state of tuning. Exhaust catalysts
offered a post-engine solution that could ensure pollutants were
converted to more benign compounds. As engine management systems and
fuel injection systems have developed, the volatility properties of
the gasoline have been tuned to minimise evaporative emissions, and
yet maintain low exhaust emissions.
The design of the engine can have very significant effects on the type
and quantity of pollutants, eg unburned hydrocarbons in the exhaust
originate mainly from combustion chamber crevices, such as the gap
between the piston and cylinder wall, where the combustion flame can
not completely use the HCs. The type and amount of unburned
hydrocarbons are related to the fuel composition (volatility, olefins,
aromatics, final boiling point), as well as state of tune, engine
condition, and age/condition of the engine lubricating oil [46].
Particulate emissions, especially the size fraction smaller than ten
micrometres, are a serious health concern. The current major source is
from compression ignition ( CI = diesel ) engines, and the modern SI
engine system has no problem meeting regulatory requirements.
The ability of reformulated gasolines to actually reduce smog has not
yet been confirmed. The composition changes will reduce some
compounds, and increase others, making predictions of environmental
consequences extremely difficult. Planned future changes, such as the
CAA 1997 Complex model specifications, that are based on several major
ongoing government/industry gasoline and emission research programmes,
are more likely to provide unambiguous environmental improvements. The
rules for tailpipe emissions will continue to become more stringent as
countries try to minimise local problems ( smog, toxins etc.) and
global problems ( CO2 ). Reformulation does not always lower all
emissions, as evidenced by the following aldehydes from an engine with
an adaptive learning management system [33].
FTP-weighted emission rates (mg/mi)
Gasoline Reformulated
Formaldehyde 4.87 8.43
Acetaldehyde 3.07 4.71
The type of exhaust catalyst and management system can have
significant effects on the emissions [33].
FTP-weighted emission rates. (mg/mi)
Total Aromatics Total Carbonyls
Gasoline Reformulated Gasoline Reformulated
Noncatalyst 1292.45 1141.82 174.50 198.73
Oxidation Catalyst 168.60 150.79 67.08 76.94
3-way Catalyst 132.70 93.37 23.93 23.07
Adaptive Learning 111.69 105.96 17.31 22.35
If we take the five compounds listed as toxics under the Clean Air
Act, then the beneficial effects of catalysts are obvious [33].
FTP-weighted emission rates. (mg/mi)
Benzene Formaldehyde Acrolein
Gas Reform Gas Reform Gas Reform
Noncatalyst 156.18 138.48 73.25 85.24 11.62 13.20
Oxidation Cat. 27.57 25.01 28.50 35.83 3.74 3.75
3-way Catalyst 19.39 15.69 7.27 7.61 1.11 0.74
Adaptive Learn. 19.77 20.39 4.87 8.43 0.81 1.16
Acetaldehyde 1,3 Butadiene
Gas Reform Gas Reform
Noncatalyst 19.74 21.72 2.96 1.81
Oxidation Cat. 11.15 11.76 0.02 0.33
3-way Catalyst 4.43 3.64 0.07 0.05
Adaptive Learn. 3.07 4.71 0.00 0.14
The author reports analytical problems with the 1,3 Butadiene, and
only Noncatalyst values are considered reliable.
Emission Standards
There are several bodies responsible for establishing standards, and
they promulgate test cycles, analysis procedures, and the % of new
vehicles that must comply each year. The test cycles and procedures do
change ( usually indicated by an anomalous increase in the numbers in
the table ), and I have not listed the percentages of the vehicle
fleet that are required to comply. This table is only intended to
convey where we have been, and where we are going. It does not cover
any regulation in detail - readers are advised to refer to the
relevant regulations. Additional limits for other pollutants, such as
formaldehyde and particulates, are omitted. The 1994 tests signal the
transition from 50,000 to 75,000 mile compliance testing, and I have
not listed the subsequent 50,000 mile limits [47,48].
Year Federal California
HCs CO NOx Evap HCs CO NOx Evap
g/mi g/mi g/mi g/test g/mi g/mi g/mi g/test
Before regs 10.6 84.0 4.1 47 10.6 84.0 4.1 47
add crankcase +4.1 +4.1
1966 6.3 51.0 6.0
1968 6.3 51.0 6.0
1970 4.1 34.0 4.1 34.0 6
1971 4.1 34.0 4.1 34.0 4.0 6
1972 3.0 28.0 2.9 34.0 3.0 2
1973 3.0 28.0 3.0 2.9 34.0 3.0 2
1974 3.0 28.0 3.0 2.9 34.0 2.0 2
1975 1.5 15.0 3.1 2 0.90 9.0 2.0 2
1977 1.5 15.0 2.0 2 0.41 9.0 1.5 2
1980 0.41 7.0 2.0 6 0.41 9.0 1.0 2
1981 0.41 3.4 1.0 2 0.39 7.0 0.7 2
1993 0.41 3.4 1.0 2 0.25 3.4 0.4 2
1994 50,000 0.26 3.4 0.3 ? TLEV 0.13 3.4 0.4
1994 75,000 0.31 4.2 0.6 ?
1997 LEV 0.08 3.4 0.2
1997 ULEV 0.04 1.7 0.2
1998 ZEV 0.0 0.0 0.0
2004 0.13 1.8 0.16 ?
It's also worth noting that exhaust catalysts also emit platinum, and
the soluble platinum salts are some of the most potent sensitizers
known. Early research [49] reported the presence of 10% water-soluble
platinum in the emissions, however later work on monolithic catalysts
has determined the quantities of water soluble platinum emissions are
negligible [50]. The particle size of the emissions has also been
determined, and the emissions have been correlated with increasing
vehicle speed. Increasing speed also increases the exhaust gas
temperature and velocity, indicating the emissions are probably a
consequence of physical attrition.
Estimated Fuel Median Aerodynamic
Speed Consumption Emissions Particle Diameter
km/h l/100km ng/m-3 um
60 7 3.3 5.1
100 8 11.9 4.2
140 10 39.0 5.6
US Cycle-75 6.4 8.5
Using the estimated fuel consumption, and about 10m3 of exhaust gas
per litre of gasoline, the emissions are 2-40ng/km. These are 2-3
orders of magnitude lower than earlier reported work on pelletised
catalysts. These emissions may be controlled directly in the future.
They are currently indirectly controlled by the cost of platinum, and
the new requirement for the catalyst to have an operational life of at
least 100,000 miles.
5.6 Why do exhaust catalysts influence fuel composition?
Modern adaptive learning engine management systems control the
combustion stoichiometry by monitoring various ambient and engine
parameters, including exhaust gas recirculation rates, the air flow
sensor, and exhaust oxygen sensor outputs, This closed loop system
using the oxygen sensor can compensate for changes in fuel content and
air density. The oxygen sensor is also known as the lambda sensor,
because the stoichiometric mass Air/Fuel ratio is known as lambda.
Typical stoichiometric air/fuel ratios are [51]:-
6.4 methanol
9.0 ethanol
11.7 MTBE
12.1 ETBE, TAME
14.6 gasoline without oxygenates
The engine management system rapidly switches the stoichiometry
between slightly rich and slightly lean, except under wide open
throttle conditions - when the system runs open loop. The response of
the oxygen sensor to composition changes is about 3 ms, and closed
loop switching is typically 1-3 times a second, going between 50mV (
lambda = 1.05 (Lean)) to 900mV (lambda = 0.99 ( Rich)). The catalyst
oxidises about 80% of the H2, CO, and HCs, and reduces the NOx [47].
Typical reactions that occur in a modern 3-way catalyst are:-
2H2 + O2 -> 2H2O
2CO + O2 -> 2CO2
CxHy + (x + (y/4))O2 -> xCO2 + (y/2)H2O
2CO + 2NO -> N2 + 2CO2
CxHy + 2(x + (y/4))NO -> (x + (y/4))N2 + (y/2)H2O + xCO2
2H2 + 2NO -> N2 + 2H2O
CO + H20 -> CO2 + H2
CxHy + xH2O -> xCO + (x + (y/2))H2
The use of exhaust catalysts have resulted in reaction pathways that
can accidentally be responsible for increased pollution. An example is
the CARB-mandated reduction of fuel sulfur. A change from 450ppm to
50ppm, which will reduce HC & CO emissions by 20%, may increase
formaldehyde by 45% [19].
The requirement that the exhaust catalysts must now endure for 10
years or 100,000 miles will also encourage automakers to push for
lower levels of known catalyst "poisons" such as sulfur and phosphorus
in both the gasoline and lubricant. Modern catalysts are unable to
reduce the relatively high levels of NOx that are produced during lean
operation down to approved levels, thus preventing the application of
lean-burn engine technology. Recently Mazda has announced they have
developed a "lean burn" catalyst, which may enable automakers to move
the fuel combustion towards the lean side, and different gasoline
properties may be required to optimise the combustion and reduce
pollution. Mazda claim that fuel efficiency is improved by 5-8% while
meeting all emission regulations [52] .
Catalysts also inhibit the selection of gasoline octane-improving and
cleanliness additives ( such as MMT and phosphorus-containing
additives ) that may result in refractory compounds known to
physically coat the catalyst and increase pollution.
5.7 Why are "cold start" emissions so important?
The catalyst requires heat to reach the temperature ( >300-350C )
where it functions most efficiently, and the delay until it reaches
operating temperature can produce more hydrocarbons than would be
produced during the remainder of many typical urban short trips. It
has been estimated that 70-80% of the non-methane HCs that escape
conversion by the catalysts are emitted during the first two minutes
after a cold start. As exhaust emissions have been reduced, the
significance of the evaporative emissions increases. Several
engineering techniques are being developed, including the Ford Exhaust
Gas Igniter ( uses a flame to heat the catalyst - lots of potential
problems ), zeolite hydrocarbon traps, and relocation of the catalyst
closer to the engine [47].
Reduced gasoline volatility and composition changes, along with
cleanliness additives and engine management systems, can help minimise
cold start emissions, but currently the most effective technique
appears to be rapid, deliberate heating of the catalyst, and the new
generation of low thermal inertia "fast light-up" catalysts reduce the
problem, but further research is necessary [53].
As the evaporative emissions are also starting to be reduced, the
emphasis has shifted to the refuelling emissions. These will be mainly
controlled on the vehicle, and larger canisters may be used to trap
the vapours emitted during refuelling.
5.8 When will the emissions be "clean enough"?
The California ZEV regulations effectively preclude IC vehicles,
because they stipulate zero emissions. However, the concept of
regulatory forcing of alternative vehicle propulsion technology may
have to be modified to include hybrid or fuel-cell vehicles, as the
major failing of EVs remains the lack of a cheap, light, safe, and
easily-rechargeable electrical storage device [54,55]. There are
several major projects intending to further reduce emissions from
automobiles, mainly focusing on vehicle mass and engine fuel
efficiency, but gasoline specifications and alternative fuels are also
being investigated. It may be that changes to IC engines and gasolines
will enable the IC engine to continue well into the 21st century as
the prime motive force for personal transportation.
5.9 Why are only some gasoline compounds restricted?
The less volatile hydrocarbons in gasoline are not released in
significant quantities during normal use, and the more volatile
alkanes are considerably less toxic than many other chemicals
encountered daily. The newer gasoline additives also have potentially
undesirable properties before they are even combusted. Most
hydrocarbons are very insoluble in water, with the lower aromatics
being the most soluble, however the addition of oxygen to hydrocarbons
significantly increases the mutual solubility with water.
Compound in Water Water in Compound
% mass/mass _at_ C % mass/mass _at_ C
normal decane 0.0000052 25 0.0072 25
iso-octane 0.00024 25 0.0055 20
normal hexane 0.00125 25 0.0111 20
cyclohexane 0.0055 25 0.010 20
1-hexene 0.00697 25 0.0477 30
toluene 0.0515 25 0.0334 25
benzene 0.1791 25 0.0635 25
methanol complete 25 complete 25
ethanol complete 25 complete 25
MTBE 4.8 20 1.4 20
TAME - 0.6 20
The concentrations and ratios of benzene, toluene, ethyl benzene, and
xylenes ( BTEX ) in water are often used to monitor groundwater
contamination from gasoline storage tanks or pipelines. The oxygenates
and other new additives may increase the extent of water and soil
pollution by acting as co-solvents for HCs.
Various government bodies ( EPA, OSHA, NIOSH ) are charged with
ensuring people are not exposed to unacceptable chemical hazards, and
maintain ongoing research into the toxicity of liquid gasoline
contact, water and soil pollution, evaporative emissions, and tailpipe
emissions [56]. As toxicity is found, the quantities in gasoline of
the specific chemical ( benzene ), or family of chemicals ( alkyl
leads, aromatics, olefins ) are regulated.
The recent dramatic changes caused by the need to reduce alkyl leads,
halogens, olefins, aromatics has resulted in whole new families of
compounds ( ethers, alcohols ) being introduced into fuels without
prior detailed toxicity studies being completed. If adverse results
appear, these compounds are also likely to be regulated to protect
people and the environment.
Also, as the chemistry of emissions is unravelled, the chemical
precursors to toxic tailpipe emissions ( such as higher aromatics that
produce benzene emissions ) are also controlled, even if they are not
toxic.
5.10 What does "renewable" fuel/oxygenate mean?
The general definition of "renewable" is that the carbon originates
from recent biomass, and thus does not contribute to the increased CO2
emissions. A truly "long-term" view could claim that fossil fuels are
"renewable" on a 100 million year timescale :-). There is currently a
major battle between the ethanol/ETBE lobby ( agricultural, corn
growing ), and the methanol/MTBE lobby ( oil company, petrochemical )
over an EPA mandate demanding that a specific percentage of the
oxygenates in gasoline are produced from "renewable" sources [57].
Unfortunately, "renewable" ethanol is not cost competitive when crude
oil is $18/bbl, so a federal subsidy ( $0.54/US Gallon ) and
additional state subsidies ( 11 states - from $0.08(Michigan) to
$0.66(Tenn.)/US Gal.) are provided. A judgement on the use of
"renewable" oxygenates is expected in early 1995.
5.11 Will oxygenated gasoline damage my vehicle?
The following comments assume that your vehicle was designed to
operate on unleaded, if not, then damage like valve seat recession may
also occur. Damage should not occur if the gasoline is correctly
formulated, and you select the appropriate octane, but oxygenated
gasoline will hurt your pocket. In the first year of mandated
oxygenates, it appears some refiners did not carefully formulate their
oxygenated gasoline, and driveability and emissions problems occurred.
Most reputable brands are now carefully formulated. Some older
activated carbon canisters may not function efficiently with
oxygenated gasolines, but this is a function of the type of carbon
used. How your vehicle responds to oxygenated gasoline depends on the
engine management system and state of tune. A modern system will
automatically compensate for all of the currently-permitted oxygenate
levels, thus your fuel consumption will increase. Older,
poorly-maintained, engines may require a tune up to maintain
acceptable driveability.
Be prepared to try several different brands of reformulated gasolines
to identify the most suitable brand for your vehicle, and be prepared
to change again with the seasons. This is because the refiners can
choose the oxygenate they use to meet the regulations, and may choose
to set some fuel properties, such as volatility, differently to their
competitors.
Most stories of corrosion etc, are derived from anhydrous methanol
corrosion of light metals (aluminum, magnesium), however the addition
of either 0.5% water to pure methanol, or corrosion inhibitors to
methanol/gasoline blends will prevent this. If you observe corrosion,
talk to your gasoline supplier. Oxygenated fuels may either swell or
shrink some elastomers on older cars, depending on the aromatic and
olefin content of the fuels. Cars later than 1990 should not
experience compatibility problems, and cars later than 1994 should not
experience driveability problems, but they will experience increased
fuel consumption, depending on the state of tune and engine management
system.
5.12 What does "reactivity" of emissions mean?
The traditional method of exhaust regulations was to specify the
actual HC, CO, NOx, and particulate contents. With the introduction of
oxygenates and reformulated gasolines, the volatile organic carbon
(VOC) species in the exhaust also changed. The "reactivity" refers to
the ozone-forming potential of the VOC emissions when they react with
NOx, and is being introduced as a regulatory means of ensuring that
automobile emissions do actually reduce smog formation. The
ozone-forming potential of chemicals is defined as the number of
molecules of ozone formed per VOC carbon atom, and this is called the
Incremental Reactivity. Typical values ( big is bad :-) ) are [45]:
Maximum Incremental Reactivities as mg Ozone / mg VOC
carbon monoxide 0.054
alkanes methane 0.0148
ethane 0.25
propane 0.48
n-butane 1.02
olefins ethylene 7.29
propylene 9.40
1,3 butadiene 10.89
aromatics benzene 0.42
toluene 2.73
meta-xylene 8.15
1,3,5-trimethyl benzene 10.12
oxygenates methanol 0.56
ethanol 1.34
MTBE 0.62
ETBE 1.98
5.13 What are "carbonyl" compounds?
Carbonyls are produced in large amounts under lean operating
conditions, especially when oxygenated fuels are used. Most carbonyls
are toxic, and the carboxylic acids can corrode metals. The emission
of carbonyls can be controlled by combustion stoichiometry and exhaust
catalysts.
Typical carbonyls are:-
- aldehydes ( containing -CHO ),
+ formaldehyde (HCHO) - which is formed in large amounts during
lean combustion of methanol [58].
+ acetaldehyde (CH2CHO) - which is formed during ethanol
combustion.
+ acrolein (CH2=CHCHO) - a very potent irritant.
- ketones ( containing C=0 ),
- acetone (CH3COCH3)
- carboxylic acids ( containing -COOH ),
+ formic acid (HCOOH) - formed during lean methanol combustion.
+ acetic acid (CH3COOH).
5.14 What are "gross polluters"?
It has always been known that the EPA emissions tests do not reflect
real world conditions. There have been several attempts to identify
vehicles on the road that do not comply with emissions standards.
Recent remote sensing surveys have demonstrated that the highest 10%
of CO emitters produce over 50% of the pollution, and the same ratio
applies for the HC emitters - which may not be the same vehicles
[59,60,61]. 20% of the CO emitters are responsible for 80% of the CO
emissions, consequently modifying gasoline composition is only one
aspect of pollution reduction. The new additives can help maintain
engine condition, but they can not compensate for out-of-tune, worn,
or tampered-with engines.
The most famous of these remote sensing systems is the FEAT ( Fuel
Efficiency Automobile Test ) team from the University of Denver [62].
This team is probably the world leader in remote sensing of auto
emissions to identify grossly polluting vehicles. The system measures
CO/CO2 ratio, and the HC/CO2 ratio in the exhaust of vehicles passing
through an infra-red light beam crossing the road 25cm above the
surface. The system also includes a video system that records the
licence plate, date, time, calculated exhaust CO, CO2, and HC. The
system is effective for traffic lanes up to 18 metres wide, however
rain, snow, and water spray can cause scattering of the beam.
Reference signals monitor such effects and, if possible, compensate.
The system has been comprehensively validated, including using
vehicles with on-board emissions monitoring instruments.
They can monitor up to 1000 vehicles an hour and, as an example,they
were invited to Provo, Utah to monitor vehicles, and gross polluters
would be offered free repairs [63]. They monitored over 10,000
vehicles and mailed 114 letters to owners of vehicles newer than 1965
that had demonstrated high CO levels. They received 52 responses and
repairs started in Dec 1991, and continued to Mar 1992. They offered
to purchase two vehicles at blue book price. They were declined, and
so attempted to modify those vehicles, even though their condition did
not justify the expense.
The entire monitored fleet at Provo (Utah) during Winter 1991/1992
Model year Grams CO/gallon Number of
(Median value) (mean value) Vehicles
92 40 80 247
91 55 1222
90 75 1467
89 80 1512
88 85 1651
87 90 1439
86 100 300 1563
85 120 1575
84 125 1206
83 145 719
82 170 639
81 230 612
80 220 500 551
79 350 667
78 420 584
77 430 430
76 770 317
75 760 950 163
Pre 75 920 1060 878
As observed elsewhere, over half the CO was emitted by about 10% of
the vehicles. If the 47 worst polluting vehicles were removed, that
achieves more than removing the 2,500 lowest emitting vehicles from
the total tested fleet.
Surveys of vehicle populations have demonstrated that emissions
systems had been tampered with on over 40% of the gross polluters, and
an additional 20% had defective emission control equipment [64]. No
matter what changes are made to gasoline, if owners "tune" their
engines for power, then the majority of such "tuned" vehicle will
become gross polluters. Professional repairs to gross polluters
usually improves fuel consumption, resulting in a low cost to owners (
$32/pa/Ton CO year ). The removal of CO in the Provo example above was
costed at $200/Ton CO, compared to Inspection and Maintenance programs
($780/Ton CO ), and oxygenates ( $1034-$1264/Ton CO in Colorado 1991-2
), and UNOCALs vehicle scrapping programme ( $1025/Ton of all
pollutants ).
Thus, identifying and repairing or removing gross polluters can be far
more cost-effective than playing around with reformulated gasolines
and oxygenates.
Subject: 6. What do Fuel Octane ratings really indicate?
6.1 Who invented Octane Ratings?
Since 1912 the spark ignition internal combustion engine's compression
ratio had been constrained by the unwanted "knock" that could rapidly
destroy engines. "Knocking" is a very good description of the sound
heard from an engine using fuel of too low octane. The engineers had
blamed the "knock" on the battery ignition system that was added to
cars along with the electric self-starter. The engine developers knew
that they could improve power and efficiency if knock could be
overcome.
Kettering assigned Thomas Midgley, Jr. to the task of finding the
exact cause of knock [16]. They used a Dobbie-McInnes manograph to
demonstrate that the knock did not arise from preignition, as was
commonly supposed, but arose from a violent pressure rise _after_
ignition. The manograph was not suitable for further research, so
Midgley and Boyd developed a high-speed camera to see what was
happening. They also developed a "bouncing pin" indicator that
measured the amount of knock [7]. Ricardo had developed an alternative
concept of HUCF ( Highest Useful Compression Ratio ) using a
variable-compression engine. His numbers were not absolute, as there
were many variables, such as ignition timing, cleanliness, spark plug
position, engine temperature. etc.
In 1926 Graham Edgar suggested using two hydrocarbons that could be
produced in sufficient purity and quantity [9]. These were "normal
heptane", that was already obtainable in sufficient purity from the
distillation of Jeffrey pine oil, and " an octane, named
2,4,4-trimethyl pentane " that he first synthesized. Today we call it
" iso-octane " or 2,2,4-trimethyl pentane. The octane had a high
anti-knock value, and he suggested using the ratio of the two as a
reference fuel number. He demonstrated that all the commerciallyavailable
gasolines could be bracketed between 60:40 and 40:60 parts
by volume heptane:iso-octane.
The reason for using normal heptane and iso-octane was because they
both have similar volatility properties, specifically boiling point,
thus the varying ratios 0:100 to 100:0 should not exhibit large
differences in volatility that could affect the rating test.
Heat of
Melting Point Boiling Point Density Vaporisation
C C g/ml MJ/kg
normal heptane -90.7 98.4 0.684 0.365 _at_ 25C
iso octane -107.45 99.3 0.6919 0.308 _at_ 25C
Having decided on standard reference fuels, a whole range of engines
and test conditions appeared, but today the most common are the
Research Octane Number ( RON ), and the Motor Octane Number ( MON ).
6.2 Why do we need Octane Ratings?
To obtain the maximum energy from the gasoline, the compressed
fuel/air mixture inside the combustion chamber needs to burn evenly,
propagating out from the spark plug until all the fuel is consumed.
This would deliver an optimum power stroke. In real life, a series of
pre-flame reactions will occur in the unburnt "end gases" in the
combustion chamber before the flame front arrives. If these reactions
form molecules or species that can autoignite before the flame front
arrives, knock will occur [13,14].
Simply put, the octane rating of the fuel reflects the ability of the
unburnt end gases to resist spontaneous autoignition under the engine
test conditions used. If autoignition occurs, it results in an
extremely rapid pressure rise, as both the desired spark-initiated
flame front, and the undesired autoignited end gas flames are
expanding. The combined pressure peak arrives slightly ahead of the
normal operating pressure peak, leading to a loss of power and
eventual overheating. The end gas pressure waves are superimposed on
the main pressure wave, leading to a sawtooth pattern of pressure
oscillations that create the "knocking" sound.
The combination of intense pressure waves and overheating can induce
piston failure in a few minutes. Knock and preignition are both
favoured by high temperatures, so one may lead to the other. Under
high-speed conditions knock can lead to preignition, which then
accelerates engine destruction [17].
6.3 What fuel property does the Octane Rating measure?
The fuel property the octane ratings measure is the ability of the
unburnt end gases to spontaneously ignite under the specified test
conditions. Within the chemical structure of the fuel is the ability
to withstand pre-flame conditions without decomposing into species
that will autoignite before the flame-front arrives. Different
reaction mechanisms, occurring at various stages of the pre-flame
compression stroke, are responsible for the undesirable,
easily-autoignitable, end gases.
During the oxidation of a hydrocarbon fuel, the hydrogen atoms are
removed one at a time from the molecule by reactions with small
radical species (such as OH and HO2), and O and H atoms. The strength
of carbon-hydrogen bonds depends on what the carbon is connected to.
Straight chain HCs such as normal heptane have secondary C-H bonds
that are significantly weaker than the primary C-H bonds present in
branched chain HCs like iso-octane [13,14].
The octane rating of hydrocarbons is determined by the structure of
the molecule, with long, straight hydrocarbon chains producing large
amounts of easily-autoignitable pre-flame decomposition species, while
branched and aromatic hydrocarbons are more resistant. This also
explains why the octane ratings of paraffins consistently decrease
with carbon number. In real life, the unburnt "end gases" ahead of the
flame front encounter temperatures up to about 700C due to piston
motion and radiant and conductive heating, and commence a series of
pre-flame reactions. These reactions occur at different thermal
stages, with the initial stage ( below 400C ) commencing with the
addition of molecular oxygen to alkyl radicals, followed by the
internal transfer of hydrogen atoms within the new radical to form an
unsaturated, oxygen-containing species. These new species are
susceptible to chain branching involving the HO2 radical during the
intermediate temperature stage (400-600C), mainly through the
production of OH radicals. Above 600C, the most important reaction
that produces chain branching is the reaction of one hydrogen atom
radical with molecular oxygen to form O and OH radicals.
The addition of additives such as alkyl lead and oxygenates can
significantly affect the pre-flame reaction pathways. Anti-knock
additives work by interfering at different points in the pre-flame
reactions, with the oxygenates retarding undesirable low temperature
reactions, and the alkyl lead compounds react in the intermediate
temperature region to deactivate the major undesirable chain branching
sequence [13,14].
The antiknock ability is related to the "autoignition temperature" of
the hydrocarbons. Antiknock ability is _not_ substantially related
to:-
- The energy content of fuel, this should be obvious, as oxygenates
have lower energy contents, but high octanes.
- The flame speed of the conventionally ignited mixture, this should
be evident from the similarities of the two reference
hydrocarbons. Although flame speed does play a minor part, there
are many other factors that are far more important. ( such as
compression ratio, stoichiometry, combustion chamber shape,
chemical structure of the fuel, presence of antiknock additives,
number and position of spark plugs, turbulence etc.) Flame speed
does not correlate with octane.
6.4 Why are two ratings used to obtain the pump rating?
The correct name for the (RON+MON)/2 formula is the "antiknock index",
and it remains the most important quality criteria for motorists [25].
The initial octane method developed in the 1920s was the Motor Octane
method and, over several decades, a large number of octane test
methods appeared. These were variations to either the engine design,
or the specified operating conditions [65]. During the 1950-1960s
attempts were made to internationally standardise and reduce the
number of Octane Rating test procedures.
During the late 1930s - mid 1960s, the Research method became the
important rating because it more closely represented the octane
requirements of the motorist using the fuels/vehicles/roads then
available. In the late 1960s German automakers discovered their
engines were destroying themselves on long Autobahn runs, even though
the Research Octane was within specification. They discovered that
either the MON or the Sensitivity ( the numerical difference between
the RON and MON numbers ) also had to be specified. Today it is
accepted that no one octane rating covers all use. In fact, during
1994, there have been increasing concerns in Europe about the high
Sensitivity of some commercially-available unleaded fuels.
The design of the engine and car significantly affect the fuel octane
requirement for both RON and MON. In the 1930s, most vehicles would
run on the specified Research Octane fuel, almost regardless of the
Motor Octane, whereas most 1990s engines have a 'severity" of one,
which means the engine is unlikely to knock if a changes of one RON is
matched by an equal and opposite change of MON [19].
6.5 What does the Motor Octane rating measure?
The conditions of the Motor method represent severe, sustained high
speed, high load driving. For most hydrocarbon fuels, including those
with either lead or oxygenates, the motor octane number (MON) will be
lower than the research octane number (RON).
Test Engine conditions Motor Octane
Test Method ASTM D2700-92 [66]
Engine Cooperative Fuels Research ( CFR )
Engine RPM 900 RPM
Intake air temperature 38 C
Intake air humidity 3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature 149 C
Coolant temperature 100 C
Oil Temperature 57 C
Ignition Advance - variable Varies with compression ratio
( eg 14 - 26 degrees BTDC )
Carburettor Venturi 14.3 mm
6.6 What does the Research Octane rating measure?
The Research method settings represent typical mild driving, without
consistent heavy loads on the engine.
Test Engine conditions Research Octane
Test Method ASTM D2699-92 [67]
Engine Cooperative Fuels Research ( CFR )
Engine RPM 600 RPM
Intake air temperature Varies with barometric pressure
( eg 88kPa = 19.4C, 101.6kPa = 52.2C )
Intake air humidity 3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature Not specified
Coolant temperature 100 C
Oil Temperature 57 C
Ignition Advance - fixed 13 degrees BTDC
Carburettor Venturi Set according to engine altitude
( eg 0-500m=14.3mm, 500-1000m=15.1mm )
6.7 Why is the difference called "sensitivity"?
RON - MON = Sensitivity. Because the two test methods use different
test conditions, especially the intake mixture temperatures and engine
speeds, then a fuel that is sensitive to changes in operating
conditions will have a larger difference between the two rating
methods. Modern fuels typically have sensitivities around 10. The US
87 (RON+MON/2) unleaded gasoline is required to have a 82+ MON, thus
preventing very high sensitivity fuels [25].
6.8 What sort of engine is used to rate fuels?
Automotive octane ratings are determined in a special single-cylinder
engine with a variable compression ratio ( CR 4:1 to 18:1 ) known as a
Cooperative Fuels Research ( CFR ) engine. The cylinder bore is
82.5mm, the stroke is 114.3mm, giving a displacement of 612 cm3. The
piston has four compression rings, and one oil control ring. The
intake valve is shrouded. The head and cylinder are one piece, and can
be moved up and down to obtain the desired compression ratio. The
engines have a special four-bowl carburettor that can adjust
individual bowl air/fuel ratios. This facilitates rapid switching
between reference fuels and samples. A magnetorestrictive detonation
sensor in the combustion chamber measures the rapid changes in
combustion chamber pressure caused by knock, and the amplified signal
is measured on a "knockmeter" with a 0-100 scale [66,67]. A complete
Octane Rating engine system costs about $200,000 with all the services
installed. Only one company manufactures these engines, the Waukesha
Engine Division of Dresser Industries, Waukesha. WI 53186.
6.9 How is the Octane rating determined?
To rate a fuel, the engine is set to an appropriate compression ratio
that will produce a knock of about 50 on the knockmeter for the sample
when the air/fuel ratio is adjusted on the carburettor bowl to obtain
maximum knock. Normal heptane and iso-octane are known as primary
reference fuels. Two blends of these are made, one that is one octane
number above the expected rating, and another that is one octane
number below the expected rating. These are placed in different bowls,
and are also rated with each air/fuel ratio being adjusted for maximum
knock. The higher octane reference fuel should produce a reading
around 30-40, and the lower reference fuel should produce a reading of
60-70. The sample is again tested, and if it does not fit between the
reference fuels, further reference fuels are prepared, and the engine
readjusted to obtain the required knock. The actual fuel rating is
interpolated from the knockmeter readings [66,67].
6.10 What is the Octane Distribution of the fuel?
The combination of vehicle and engine can result in specific
requirements for octane that depend on the fuel. If the octane is
distributed differently throughout the boiling range of a fuel, then
engines can knock on one brand of 87 (RON+MON/2), but not on another
brand. This "octane distribution" is especially important when sudden
changes in load occur, such as high load, full throttle, acceleration.
The fuel can segregate in the manifold, with the very volatile
fraction reaching the combustion chamber first and, if that fraction
is deficient in octane, then knock will occur until the less volatile,
higher octane fractions arrive [17].
Some fuel specifications include delta RONs, to ensure octane
distribution throughout the fuel boiling range was consistent. Octane
distribution was seldom a problem with the alkyl lead compounds, as
the tetra methyl lead and tetra ethyl lead octane volatility profiles
were well characterised, but it can be a major problem for the new,
reformulated, low aromatic gasolines, as MTBE boils at 55C, whereas
ethanol boils at 78C. Drivers have discovered that an 87 (RON+MON/2)
from one brand has to be substituted with an 89 (RON+MON/2) of
another, and that is because of the combination of their driving
style, engine design, vehicle mass, fuel octane distribution, fuel
volatility, and the octane-enhancers used.
6.11 What is a "delta Research Octane number"?
To obtain an indication of behaviour of a gasoline during any manifold
segregation, an octane rating procedure called the Distribution Octane
Number was used. The rating engine had a special manifold that allowed
the heavier fractions to be separated before they reached the
combustion chamber [17]. That method has been replaced by the "delta"
RON procedure.
The fuel is carefully distilled to obtain a distillate fraction that
boils to the specified temperature, which is usually 100C. Both the
parent fuel and the distillate fraction are rated on the octane engine
using the Research Octane method [68]. The difference between these is
the delta RON(100C), usually just called the delta RON.
6.12 How do other fuel properties affect octane?
Several other properties affect knock. The most significant
determinant of octane is the chemical structure of the hydrocarbons
and their response to the addition of octane enhancing additives.
Other factors include:-
- Front End Volatility - Paraffins are the major component in
gasoline, and the octane number decreases with increasing chain
length or ring size, but increases with chain branching. Overall,
the effect is a significant reduction in octane if front end
volatility is lost, as can happen with improper or long term
storage. Fuel economy on short trips can be improved by using a
more volatile fuel, at the risk of carburettor icing and increased
evaporative emissions.
- Final Boiling Point.- Decreases in the final boiling point
increase fuel octane. Aviation gasolines have much lower final
boiling points than automotive gasolines. Note that final boiling
points are being reduced because the higher boiling fractions are
responsible for disproportionate quantities of pollutants and
toxins.
- Preignition tendency - both knock and preignition can induce each
other.
6.13 Can higher octane fuels give me more power?
Not if you are already using the proper octane fuel. The engine will
be already operating at optimum settings, and a higher octane should
have no effect on the management system. Your driveability and fuel
economy will remain the same. The higher octane fuel costs more, so
you are just throwing money away. If you are already using a fuel with
an octane rating slightly below the optimum, then using a higher
octane fuel will cause the engine management system to move to the
optimum settings, possibly resulting in both increased power and
improved fuel economy. You may be able to change octanes between
seasons ( reduce octane in winter ) to obtain the most cost-effective
fuel without loss of driveability.
Once you have identified the fuel that keeps the engine at optimum
settings, there is no advantage in moving to an even higher octane
fuel. The manufacturer's recommendation is conservative, so you may be
able to carefully reduce the fuel octane. The penalty for getting it
badly wrong, and not realising that you have, could be expensive
engine damage.
6.14 Does low octane fuel increase engine wear?
Not if you are meeting the octane requirement of the engine. If you
are not meeting the octane requirement, the engine will rapidly suffer
major damage due to knock. You must not use fuels that produce
sustained audible knock, engine damage will occur. If the octane is
just sufficient, the engine management system will move settings to a
less optimal position, and the only major penalty will be increased
costs due to poor fuel economy. Whenever possible, engines should be
operated at the optimum position for long-term reliability. Engine
wear is mainly related to design, manufacturing, maintenance and
lubrication factors. Once the octane and run-on requirements of the
engine are satisfied, increased octane will have no beneficial effect
on the engine. The quality of gasoline, and the additive package used,
would be more likely to affect the rate of engine wear, rather than
the octane rating.
6.15 Can I mix different octane fuel grades?
Yes, however attempts to blend in your fuel tank should be carefully
planned. You should not allow the tank to become empty, and then add
50% of lower octane, followed by 50% of higher octane. The fuels may
not completely mix immediately, especially if there is a density
difference. You may get a slug of low octane that causes severe knock.
You should refill when your tank is half full. In general the octane
response will be linear for most hydrocarbon and oxygenated fuels eg
50:50 of 87 and 91 will give 89.
Attempts to mix leaded high octane to unleaded high octane to obtain
higher octane are useless. The lead response of the unleaded fuel does
not overcome the dilution effect, thus 50:50 of 96 leaded and 91
unleaded will give 94. Some blends of oxygenated fuels with ordinary
gasoline can result in undesirable increases in volatility due to
volatile azeotropes, and that some oxygenates can have negative lead
responses. Also note that the octane requirement of some engines is
determined by the need to avoid run-on, not to avoid knock.
6.16 What happens if I use the wrong octane fuel?
If you use a fuel with an octane rating below the requirement of the
engine, the management system may move the engine settings into an
area of less efficient combustion, resulting in reduced power and
reduced fuel economy. You will be losing both money and driveability.
If you use a fuel with an octane rating higher than what the engine
can use, you are just wasting money by paying for octane that you can
not utilise. Forget the stories about higher octanes having superior
additive packages - they do not. If your vehicle does not have a knock
sensor, then using an octane significantly below the requirement means
that the little men with hammers will gleefully pummel your engine to
pieces.
You should initially be guided by the vehicle manufacturer's
recommendations, however you can experiment, as the variations in
vehicle tolerances can mean that Octane Number Requirement for a given
vehicle model can range over 6 Octane Numbers. Caution should be used,
and remember to compensate if the conditions change, such as carrying
more people or driving in different ambient conditions. You can often
reduce the octane of the fuel you use in winter because the
temperature decrease and possible humidity changes may significantly
reduce the octane requirement of the engine.
Use the octane that provides cost-effective driveability and
performance, using anything more is waste of money, and anything less
could result in an unscheduled, expensive visit to your mechanic.
6.17 Can I tune the engine to use another octane fuel?
In general, modern engine management systems will compensate for fuel
octane, and once you have satisfied the optimum octane requirement,
you are at the optimum overall performance area of the engine map.
Tuning changes to obtain more power will probably adversely affect
both fuel economy and emissions. Unless you have access to good
diagnostic equipment that can ensure regulatory limits are complied
with, it is likely that adjustments may be regarded as illegal
tampering by your local regulation enforcers. If you are skilled, you
will be able to legally wring slightly more performance from your
engine by using a dynamometer in conjunction with engine and exhaust
gas analyzers and a well-designed, retrofitted, performance engine
management chip.
6.18 How can I increase the fuel octane?
Not simply, you can purchase additives, however these are not
cost-effective and a survey in 1989 showed the cost of increasing the
octane rating of one US gallon by one unit ranged from 10 cents (
methanol ), 50 cents (MMT), $1.00 ( TEL ), to $3.25 ( xylenes ) [69].
It is preferable to purchase a higher octane fuel such as racing fuel,
aviation gasolines, or methanol. Sadly, the price of chemical grade
methanol has almost doubled during 1994. If you plan to use alcohol
blends, ensure your fuel handling system is compatible, and that you
only use dry gasoline by filling up early in the morning when the
storage tanks are cool. Also ensure that the service station storage
tank has not been refilled recently. Retailers are supposed to wait
several hours before bringing a refilled tank online, to allow
suspended undissolved water to settle out, but they do not always wait
the full period.
6.19 Are aviation gasoline octane numbers comparable?
Aviation gasolines were all highly leaded and graded using two
numbers, with common grades being 80/87, 100/130, and 115/145 [70].
The first number is the Aviation rating ( aka Lean Mixture rating ),
and the second number is the Supercharge rating ( aka Rich Mixture
rating ). In the 1970s a new grade, 100LL ( low lead = 0.53mlTEL/L
instead of 1.06mlTEL/L) was introduced to replace the 80/87 and
100/130. Soon after the introduction, there was a spate of plug
fouling, and high cylinder head temperatures resulting in cracked
cylinder heads [71]. The old 80/87 grade was reintroduced on a limited
scale. The Aviation rating is determined using the automotive Motor
Octane test procedure, and then corrected to an Aviation number using
a table in the method - it's usually only 1 - 2 Octane units different
to the Motor value up to 100, but varies significant above that eg
110MON = 128AN.
The second Avgas number is the Rich Mixture method Performance Number
( PN - they are not commonly called octane numbers when they are above
100 ), and is determined on a supercharged version of the CFR engine
which has a fixed compression ratio. The method determines the
dependence of the highest permissible power ( in terms of indicated
mean effective pressure ) on mixture strength and boost for a specific
light knocking setting. The Performance Number indicates the maximum
knock-free power obtainable from a fuel compared to iso-octane = 100.
Thus, a PN = 150 indicates that an engine designed to utilise the fuel
can obtain 150% of the knock-limited power of iso-octane at the same
mixture ratio. This is an arbitrary scale based on iso-octane +
varying amounts of TEL, derived from a survey of engines performed
decades ago. Aviation gasoline PNs are rated using variations of
mixture strength to obtain the maximum knock-limited power in a
supercharged engine. This can be extended to provide mixture response
curves which define the maximum boost ( rich - about 11:1
stoichiometry ) and minimum boost ( weak about 16:1 stoichiometry )
before knock [71].
The 115/145 grade is being phased out, but even the 100LL has more
octane than any automotive gasoline.
Subject: 7. What parameters determine octane requirement? 7.1 What is the
effect of Compression ratio?
Most people know that an increase in Compression Ratio will require an
increase in fuel octane for the same engine design. Increasing the
compression ratio increases the theoretical thermodynamic efficiency
of an engine according to the standard equation
Efficiency = 1 - (1/compression ratio)^gamma-1
where gamma = ratio of specific heats at constant pressure and
constant volume of the working fluid ( for most purposes air is the
working fluid, and is treated as an ideal gas ). There are indications
that thermal efficiency reaches a maximum at a compression ratio of
about 17:1 [15].
The efficiency gains are best when the engine is at incipient knock,
that's why knock sensors ( actually vibration sensors ) are used. Low
compression ratio engines are less efficient because they can not
deliver as much of the ideal combustion power to the flywheel. For a
typical carburetted engine, without engine management [17,24]:-
Compression Octane Number Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %
Modern engines have improved significantly on this, and the changing
fuel specifications and engine design should see more improvements,
but significant gains may have to await improved engine materials and
fuels.
7.2 What is the effect of changing the air/fuel ratio?
Traditionally, the greatest tendency to knock was near 13.5:1 air/fuel
ratio, but was very engine specific. Modern engines, with engine
management systems, now have their maximum octane requirement near to
14.5:1. For a given engine using gasoline, the relationship between
thermal efficiency, air/fuel ratio, and power is complex.
Stoichiometric combustion ( Air/Fuel Ratio = 14.7:1 for a typical
non-oxygenated gasoline ) is neither maximum power - which occurs
around A/F 12-13:1 (Rich), nor maximum thermal efficiency - which
occurs around A/F 16-18:1 (Lean). The air-fuel ratio is controlled at
part throttle by a closed loop system using the oxygen sensor in the
exhaust. Conventionally, enrichment for maximum power air/fuel ratio
is used during full throttle operation to reduce knocking while
providing better driveability [24]. If the mixture is weakened, the
flame speed is reduced, consequently less heat is converted to
mechanical energy, leaving heat in the cylinder walls and head,
potentially inducing knock. It is possible to weaken the mixture
sufficiently that the flame is still present when the inlet valve
opens again, resulting in backfiring.
7.3 What is the effect of changing the ignition timing
The tendency to knock increases as spark advance is increased, eg 2
degrees BTDC requires 91 octane, whereas 14 degrees BTDC requires 96
octane. If you advance the spark, the flame front starts earlier, and
the end gases start forming earlier in the cycle, providing more time
for the autoigniting species to form before the piston reaches the
optimum position for power delivery, as determined by the normal flame
front propagation. It becomes a race between the flame front and
decomposition of the increasingly-squashed end gases. High octane
fuels produce end gases that take longer to autoignite, so the good
flame front reaches and consumes them properly.
The ignition advance map is partly determined by the fuel the engine
is intended to use. The timing of the spark is advanced sufficiently
to ensure that the fuel/air mixture burns in such a way that maximum
pressure of the burning charge is about 15-20 degree after TDC. Knock
will occur before this point, usually in the late compression/early
power stroke period. The engine management system uses ignition timing
as one of the major variables that is adjusted if knock is detected.
If very low octane fuels are used ( several octane numbers below the
vehicle's requirement at optimal settings ), both performance and fuel
economy will decrease.
The actual Octane Number Requirement depends on the engine design, but
for some 1978 vehicles using standard fuels, the following (R+M)/2
Octane Requirements were measured. "Standard" is the recommended
ignition timing for the engine, probably a few degrees before Top Dead
Centre [24].
Basic Ignition Timing
Vehicle Retarded 5 degrees Standard Advanced 5 degrees
A 88 91 93
B 86 90.5 94.5
C 85.5 88 90
D 84 87.5 91
E 82.5 87 90
The actual ignition timing to achieve the maximum pressure from normal
combustion of gasoline will depend mainly on the speed of the engine
and the flame propagation rates in the engine. Knock increases the
rate of the pressure rise, thus superimposing additional pressure on
the normal combustion pressure rise. The knock actually rapidly
resonates around the chamber, creating a series of abnormal sharp
spikes on the pressure diagram. The normal flame speed is fairly
consistent for most gasoline HCs, regardless of octane rating, but the
flame speed is affected by stoichiometry. Note that the flame speeds
in this FAQ are not the actual engine flame speeds. A 12:1 CR gasoline
engine at 1500 rpm would have a flame speed of about 16.5 m/s, and a
similar hydrogen engine yields 48.3 m/s, but such engine flame speeds
are also very dependent on stoichiometry.
7.4 What is the effect of engine management systems?
Engine management systems are now an important part of the strategy to
reduce automotive pollution. The good news for the consumer is their
ability to maintain the efficiency of gasoline combustion, thus
improving fuel economy. The bad news is their tendency to hinder
tuning for power. A very basic modern engine system could monitor and
control:- mass air flow, fuel flow, ignition timing, exhaust oxygen (
lambda oxygen sensor ), knock ( vibration sensor ), EGR, exhaust gas
temperature, coolant temperature, and intake air temperature. The
knock sensor can be either a nonresonant type installed in the engine
block and capable of measuring a wide range of knock vibrations ( 5-15
kHz ) with minimal change in frequency, or a resonant type that has
excellent signal-to-noise ratio between 1000 and 5000 rpm [72].
A modern engine management system can compensate for altitude, ambient
air temperature, and fuel octane. The management system will also
control cold start settings, and other operational parameters. There
is a new requirement that the engine management system also contain an
on-board diagnostic function that warns of malfunctions such as engine
misfire, exhaust catalyst failure, and evaporative emissions failure.
The use of fuels with alcohols such as methanol can confuse the engine
management system as they generate more hydrogen which can fool the
oxygen sensor [47] .
The use of fuel of too low octane can actually result in both a loss
of fuel economy and power, as the management system may have to move
the engine settings to a less efficient part of the performance map.
The system retards the ignition timing until only trace knock is
detected, as engine damage from knock is of more consequence than
power and fuel economy.
7.5 What is the effect of temperature and load?
Increasing the engine temperature, particularly the air/fuel charge
temperature, increases the tendency to knock. The Sensitivity of a
fuel can indicate how it is affected by charge temperature variations.
Increasing load increases both the engine temperature, and the end-gas
pressure, thus the likelihood of knock increases as load increases.
7.6 What is the effect of engine speed?.
Faster engine speed means there is less time for the pre-flame
reactions in the end gases to occur, thus reducing the tendency to
knock. On engines with management systems, the ignition timing may be
advanced with engine speed and load, to obtain optimum efficiency at
incipient knock. In such cases, both high and low engines speeds may
be critical.
7.7 What is the effect of engine deposits?
A new engine may only require a fuel of 6-9 octane numbers lower than
the same engine after 25,000 km. This Octane Requirement Increase
(ORI) is due to the formation of a mixture of organic and inorganic
deposits resulting from both the fuel and the lubricant. They reach an
equilibrium amount because of flaking, however dramatic changes in
driving styles can also result in dramatic changes of the equilibrium
position. When the engine starts to burn more oil, the octane
requirement can increase again. ORIs up to 12 are not uncommon,
depending on driving style [17,19]. The deposits produce the ORI by
several mechanisms:-
- they reduce the combustion chamber volume, effectively increasing
the compression ratio. - they also reduce thermal conductivity, thus
increasing the combustion chamber temperatures. - they catalyse
undesirable pre-flame reactions that produce end gases with low
autoignition temperatures.
7.8 What is the Road Octane requirement of an vehicle?
The actual octane requirements of a vehicle is called the Octane
Number Requirement ( ONR ), and is determined by using standard octane
fuels that can be blends of iso-octane and normal heptane, or
commercial gasolines. The vehicle is tested under a wide range of
conditions and loads, using different octane fuels until trace knock
is detected. The conditions that require maximum octane are not
consistent, but often are full-throttle acceleration from low starting
speeds using the highest gear available. They can even be at constant
speed conditions [17]. Engine management systems that adjust the
octane requirement may also reduce the power output on low octane
fuel, resulting in increased fuel consumption. The maximum ONR is of
most interest, as that usually defines the recommended fuel.
The octane rating engines do not reflect actual conditions in a
vehicle, consequently there are standard procedures for evaluating the
performance of the gasoline in an engine. The most common are:- 1. The
Modified Uniontown Procedure. Full throttle accelerations are made
from low speed using primary reference fuels. The ignition timing is
adjusted until trace knock is detected at some stage. Several
reference fuels are used, and a Road Octane Number v Basic Ignition
timing graph is obtained. The fuel sample is tested, and the ignition
timing setting is read from the graph to provide the Road Octane
Number. This is a rapid procedure but provides minimal information. 2.
The Modified Borderline Knock Procedure. The automatic spark advance
is disabled, and a manual adjustment facility added. Accelerations are
performed as in the Modified Uniontown Procedure, however trace knock
is maintained throughout the run. A map of ignition advance v engine
speed is made for several reference fuels and the sample fuels. This
procedure can show the variation of road octane with engine speed.
7.9 What is the effect of air temperature?
An increase in ambient air temperature of 5.6C increases the octane
requirement of an engine by 0.44 - 0.54 MON [17,24]. When the combined
effects of air temperature and humidity are considered, it is often
possible to use one octane grade in summer, and use a lower octane
rating in winter. The Motor octane rating has a higher charge
temperature, and increasing charge temperature increases the tendency
to knock, so fuels with low Sensitivity ( the difference between RON
and MON numbers ) are less affected by air temperature.
7.10 What is the effect of altitude?
The effect of increasing altitude may be nonlinear, with one study
reporting a decrease of the octane requirement of 1.4 RON/300m from
sea level to 1800m and 2.5 RON/300m from 1800m to 3600m [17]. Other
studies report the octane number requirement decreased by 1.0 - 1.9
RON/300m without specifying altitude [24]. Modern engine management
systems can accommodate this adjustment, and in some recent studies,
the octane number requirement was 0.2 - 0.5 Antiknock Index/300m. The
reduction on older engines was due to:-
- reduced air density provides lower combustion temperature and
pressure. - fuel is metered according to air volume, consequently as
density decreases the stoichiometry moves to rich, with a lower octane
number requirement. - manifold vacuum controlled spark advance, and
reduced manifold vacuum results in less spark advance.
7.11 What is the effect of humidity?.
An increase of absolute humidity of 1.0 g water/ kg of dry air lowers
the octane requirement of an engine by 0.25 - 0.32 MON [17,24].
7.12 What does water injection achieve?.
Water injection was used in WWII aviation engine to provide a large
increase in available power for very short periods. The injection of
water does decrease the dew point of the exhaust gases. This has
potential corrosion problems. The very high specific heat and heat of
vaporisation of water means that the combustion temperature will
decrease. It has been shown that a 10% water addition to methanol
reduces the power and efficiency by about 3%, and doubles the unburnt
fuel emissions, but does reduce NOx by 25% [73]. A decrease in
combustion temperature will reduce the theoretical maximum possible
efficiency of an otto cycle engine that is operating correctly, but
may improve efficiency in engines that are experiencing abnormal
combustion on existing fuels.
Some aviation SI engines still use boost fluids. The water/methanol
mixtures are used to provide increased power for short periods, up to
40% more - assuming adequate mechanical strength of the engine. The
40/60 or 45/55 water/methanol mixtures are used as boost fluids for
aviation engines because water would freeze. Methanol is just
"preburnt" methane, consequently it only has about half the energy
content of gasoline, but it does have a higher heat of vaporisation,
which has a significant cooling effect on the charge. Water/methanol
blends are more cost-effective than gasoline for combustion cooling.
The high Sensitivity of alcohol fuels has to be considered in the
engine design and settings.
Boost fluids are used because they are far more economical than using
the fuel. When a supercharged engine has to be operated at high boost,
the mixture has to be enriched to keep the engine operating without
knock. The extra fuel cools the cylinder walls and the charge, thus
delaying the onset of knock which would otherwise occur at the
associated higher temperatures.
The overall effect of boost fluid injection is to permit a
considerable increase in knock-free engine power for the same
combustion chamber temperature. The power increase is obtained from
the higher allowable boost. In practice, the fuel mixture is usually
weakened when using boost fluid injection, and the ratio of the two
fuel fluids is approximately 100 parts of avgas to 25 parts of boost
fluid. With that ratio, the resulting performance corresponds to an
effective uprating of the fuel of about 25%, irrespective of its
original value. Trying to increase power boosting above 40% is
difficult, as the engine can drown because of excessive liquid [71].
Note that for water injection to provide useful power gains, the
engine management and fuel systems must be able to monitor the knock
and adjust both stoichiometry and ignition to obtain significant
benefits. Aviation engines are designed to accommodate water
injection, most automobile engines are not. Returns on investment are
usually harder to achieve on engines that do not normal extend their
performance envelope into those regions. Water injection has been used
by some engine manufacturers - usually as an expedient way to maintain
acceptable power after regulatory emissions baggage was added to the
engine, but usually the manufacturer quickly produces a modified
engine that does not require water injection.
Subject: 8. How can I identify and cure other fuel-related problems? 8.1 What
causes an empty fuel tank?
- You forgot to refill it.
- Your friendly neighbourhood thief "borrowed" the gasoline - the
unfriendly one took the vehicle.
- The fuel tank leaked.
- Your darling child/wife/husband/partner/mother/father used the
car.
- The most likely reason is that your local garage switched to an
oxygenated gasoline, and the engine management system compensated
for the oxygen content, causing the fuel consumption to increase
significantly.
8.2 Is knock the only abnormal combustion problem?
No. Many of the abnormal combustion problems are induced by the same
conditions, and so one can lead to another.
Preignition occurs when the air/fuel mixture is ignited prematurely by
glowing deposits or hot surfaces - such as exhaust valves and spark
plugs. If it continues, it can increase in severity and become
Run-away Surface Ignition (RSI) which prevents the combustion heat
being converted into mechanical energy, thus rapidly melting pistons.
The Ricardo method uses an electrically-heated wire in the engine to
measure preignition tendency. The scale uses iso-octane as 100 and
cyclohexane as 0.
Some common fuel components:-
paraffins 50-100
benzene 26
toluene 93
xylene >100
cyclopentane 70
di-isobutylene 64
hexene-2 -26
There is no direct correlation between anti-knock ability and
preignition tendency, however high combustion chamber temperatures
favour both, and so one may lead to the other. An engine knocking
during high-speed operation will increase in temperature and that can
induce preignition, and conversely any preignition will result in
higher temperatures than may induce knock.
Misfire is commonly caused by either a failure in the ignition system,
or fouling of the spark plug by deposits. The most common cause of
deposits was the alkyl lead additives in gasoline, and the yellow
glaze of various lead salts was used by mechanics to assess engine
tune. From the upper recess to the tip, the composition changed, but
typical compounds ( going from cold to hot ) were PbClBr; 2PbO.PbClBr;
PbO.PbSO4; 3Pb3(PO4)2.PbClBr.
Run-on is the tendency of an engine to continue running after the
ignition has been switched off. It is usually caused by the
spontaneous ignition of the fuel/air mixture, rather than by surface
ignition from hotspots or deposits, as commonly believed. The narrow
range of conditions for spontaneous ignition of the fuel/air mixture (
engine speed, charge temperature, cylinder pressure ) may be created
when the engine is switched off. The engine may refire, thus taking
the conditions out of the critical range for a couple of cycles, and
then refire again, until overall cooling of the engine drops it out of
the critical region. The octane rating of the fuel is the appropriate
parameter, and it is not rare for an engine to require a higher Octane
fuel to prevent run-on than to avoid knock [17].
8.3 Can I prevent carburetter icing?
Yes, carburettor icing is caused by the combination of highly volatile
fuel, high humidity and low ambient temperature. The extent of
cooling, caused by the latent heat of the vaporised gasoline in the
carburettor, can be as much as 20C, perhaps dropping below the dew
point of the charge. If this happens, water will condense on the
cooler carburettor surfaces, and will freeze if the temperature is low
enough. The fuel volatility can not always be reduced to eliminate
icing, so anti-icing additives are used.
Two types of additive are added to gasoline to inhibit icing:-
- surfactants that form a monomolecular layer over the metal parts
that inhibits ice crystal formation. These are usually added at
concentrations of 30-150 ppm. - cryoscopic additives that depress the
freezing point of the condensed water so that it does not turn to ice.
Alcohols ( methanol, ethanol, iso-propanol, etc. ) and glycols (
hexylene glycol, dipropylene glycol ) are used at concentrations of
0.03% - 1%. If you have icing problems, the addition of 100-200mls of
methanol to a full tank of dry gasoline will prevent icing under most
conditions. If you believe there is a small amount of water in the
fuel tank, add 500mls of isopropanol as the first treatment.
Oxygenated gasolines using alcohols can also be used.
8.4 Should I store fuel to avoid the oxygenate season?
No. The fuel will be from a different season, and will have
significantly different volatility properties that may induce
driveability problems. You can tune your engine to perform on
oxygenated gasoline as well as it did on traditional gasoline, however
you will have increased fuel consumption due to the useless oxygen in
the oxygenates. Some engines may not initially perform well on some
oxygenated fuels, usually because of the slightly different volatility
and combustion characteristics. A good mechanic should be able to
recover any lost performance or driveability, providing the engine is
in reasonable condition.
8.5 Can I improve fuel economy by using quality gasolines?
Yes, several manufacturers have demonstrated that their new gasoline
additive packages are more effective than traditional gasoline
formulations. Texaco claim their new vapour phase fuel additive can
reduce existing deposits by up to 30%, improve fuel economy, and
reduce NOx tailpipe emissions by 15%, when compared to other advanced
liquid phase additives. These claims appear to have been verified in
independent tests [30]. Other reputable gasoline brands will have
similar additive packages in their quality products. Quality
gasolines, of whatever octane ratings, will include a full range of
gasoline additives designed to provide consistent fuel quality.
Note that oxygenated gasolines must decrease fuel economy for the same
power. If your engine is initially well-tuned on hydrocarbon
gasolines, the stoichiometry will move to lean, and maximum power is
slightly rich, so either the management system ( if you have one ) or
your mechanic has to increase the fuel flow. The minor improvements in
combustion efficiency that oxygenates may provide, can not compensate
for 2+% of oxygen in the fuel that will not provide energy.
8.6 What is "stale" fuel, and should I use it?
"Stale" fuel is caused by improper storage, and usually smells sour.
The gasoline has been allowed to get warm, thus catalysing olefin
decomposition reactions, and perhaps also losing volatile material in
unsealed containers. Such fuel will tend to rapidly form gums, and
will usually have a significant reduction in octane rating. The fuel
can be used by blending with twice the volume of new gasoline. Some
stale fuels can drop several octane numbers, so be generous with the
dilution.
8.7 How can I remove water in the fuel tank?
If you only have a small quantity of water, then the addition of
500mls of dry isopropanol (IPA) to a near-full 30-40 litre tank will
absorb the water, and will not significantly affect combustion. Once
you have mopped up the water with IPA. Small, regular doses of any
anhydrous alcohol will help keep the tank dry. This technique will not
work if you have very large amounts of water, and the addition of
greater amounts of IPA may result in poor driveability.
Water in fuel tanks can be minimised by keeping the fuel tank near
full, and filling in the morning from a service station that allows
storage tanks to stand for several hours after refilling before using
the fuel. Note that oxygenated gasolines have greater water
solubility, and should cope with small quantities of water.
8.8 Can I used unleaded on older vehicles?
Yes, providing the octane is appropriate. There are some older engines
that cut the valve seats directly into the cylinder head ( eg BMC
minis ). The absence of lead, which lubricated the valve seat, causes
the very hard oxidation products of the valve to wear down the seat.
This valve seat recession is usually corrected by installing seat
inserts. Most other problems arise because the fuels have different
volatility, or the reduction of combustion chamber deposits. These can
usually be cured by reference to the vehicle manufacturer, who will
probably have a publication with the changes. Some vehicles will
perform as well on unleaded with a slightly lower octane than
recommended leaded fuel, due to the significant reduction in deposits
from modern unleaded gasolines.
Section: 9. Alternative Fuels and Additives 9.1 Do fuel additives work?
Most aftermarket fuel additives are not cost-effective. These include
the octane-enhancer solutions discussed in section 6.18. There are
various other pills, tablets, magnets, filters, etc. that all claim to
improve either fuel economy or performance. Some of these have
perfectly sound scientific mechanisms, unfortunately they are not
cost-effective. Some do not even have sound scientific mechanisms.
Because the same model production vehicles can vary significantly,
it's expensive to unambiguously demonstrate these additives are not
cost-effective. If you wish to try them, remember the biggest gain is
likely to be caused by the lower mass of your wallet/purse.
There is one aftermarket additive that may be cost-effective, the
lubricity additive used with unleaded gasolines to combat valve seat
recession on engines that do not have seat inserts. The long-term
solution is to install inserts at the next top overhaul.
Some other fuel additives work, especially those that are carefully
formulated into the gasoline by the manufacturer at the refinery.
A typical gasoline may contain [17,19,24]:-
- Oil-soluble Dye, initially added to leaded gasoline at about 10 ppm
to prevent its misuse as an industrial solvent * Antioxidants,
typically phenylene diamines or hindered phenols, are added to prevent
oxidation of unsaturated hydrocarbons. * Metal Deactivators, typically
about 10ppm of chelating agent such as
N,N'-disalicylidene-1,2-propanediamine is added to inhibit copper,
which can rapidly catalyze oxidation of unsaturated hydrocarbons. *
Corrosion Inhibitors, about 5ppm of oil-soluble surfactants are added
to prevent corrosion caused either by water condensing from cooling,
water-saturated gasoline, or from condensation from air onto the walls
of almost-empty gasoline tanks that drop below the dew point. If your
gasoline travels along a pipeline, it's possible the pipeline owner
will add additional corrosion inhibitor to the fuel. * Anti-icing
Additives, used mainly with carburetted cars, and usually either a
surfactant, alcohol or glycol. * Anti-wear Additives, these are used
to control wear in the upper cylinder and piston ring area that the
gasoline contacts, and are usually very light hydrocarbon oils.
Phosphorus additives can also be used on engines without exhaust
catalyst systems. * Deposit-modifying Additives, usually surfactants.
- Carburettor Deposits, additives to prevent these were required when
crankcase blow-by (PCV) and exhaust gas recirculation (EGR) controls
were introduced. Some fuel components reacted with these gas streams
to form deposits on the throat and throttle plate of carburettors. 2.
Fuel Injector tips operate about 100C, and deposits form in the
annulus during hot soak, mainly from the oxidation and polymerisation
of the larger unsaturated hydrocarbons. The additives that prevent and
unclog these tips are usually polybutene succinimides or polyether
amines. 3. Intake Valve Deposits caused major problems in the
mid-1980s when some engines had reduced driveability when fully
warmed, even though the amount of deposit was below previously
acceptable limits. It is believed that the new fuels and engine
designs were producing a more absorbent deposit that grabbed some
passing fuel vapour, causing lean hesitation. Intake valves operate
about 300C, and if the valve is is kept wet deposits tend not to form,
thus intermittent injectors tend to promote deposits. Oil leaking
through the valve guides can be either harmful or beneficial,
depending on the type and quantity. Gasoline factors implicated in
these deposits include unsaturates and alcohols. Additives to prevent
these deposits contain a detergent and/or dispersant in a higher
molecular weight solvent or light oil whose low volatility keeps the
valve surface wetted. 4. Combustion Chamber Deposits have been
targeted in the 1990s, as they are responsible for significant
increases in emissions. Recent detergent-dispersant additives have the
ability to function in both the liquid and vapour phases to remove
existing carbon and prevent deposit formation. * Octane Enhancers,
these are usually formulated blends of alkyl lead or MMT compounds in
a solvent such as toluene, and added at the 100-1000 ppm levels. They
have been replaced by hydrocarbons with higher octanes such as
aromatics and olefins. These hydrocarbons are now being replaced by a
mixture of saturated hydrocarbons and and oxygenates. If you wish to
play with different fuels and additives, be aware that some parts of
your engine management systems, such as the oxygen sensor, can be
confused by different exhaust gas compositions. An example is
increased quantities of hydrogen from methanol combustion.
9.2 Can a quality fuel help a sick engine?
It depends on the ailment. Nothing can compensate for poor tuning and
wear. If the problem is caused by deposits or combustion quality, then
modern premium quality gasolines have been shown to improve engine
performance significantly. The new generation of additive packages for
gasolines include components that will dissolve existing carbon
deposits, and have been shown to improve fuel economy, NOx emissions,
and driveability.
9.3 What are the advantages of alcohols and ethers?
This section discusses only the use of high ( >80% ) alcohol or ether
fuels. Alcohol fuels can be made from sources other than imported
crude oil, and the nations that have researched/used alcohol fuels
have mainly based their choice on import substitution. Alcohol fuels
can burn more efficiently, and can reduce photochemically-active
emissions. Most vehicle manufacturers favoured the use of liquid fuels
over compressed or liquified gases. The alcohol fuels have high
research octane ratings, but also high sensitivity and high latent
heats [6,17,51,74].
Methanol Ethanol Unleaded Gasoline
RON 106 107 92 - 98
MON 92 89 80 - 90
Heat of Vaporisation (MJ/kg) 1.154 0.913 0.3044
Nett Heating Value (MJ/kg) 19.95 26.68 42 - 44
Vapour Pressure _at_ 38C (kPa) 31.9 16.0 48 - 108
Flame Temperature ( C ) 1870 1920 2030
Stoich. Flame Speed. ( m/s ) 0.43 - 0.34
Minimum Ignition Energy ( mJ ) 0.14 - 0.29
Lower Flammable Limit ( vol% ) 6.7 3.3 1.3
Upper Flammable Limit ( vol% ) 36.0 19.0 7.1
Autoignition Temperature ( C ) 460 360 260 - 460
Flash Point ( C ) 11 13 -43 - -39
The major advantages are gained when pure fuels ( M100, and E100 ) are
used, as the addition of hydrocarbons to overcome the cold start
problems also significantly reduces, if not totally eliminates, any
emission benefits. Methanol will produce significant amounts of
formaldehyde, a suspected human carcinogen, until the exhaust catalyst
reaches operating temperature. Ethanol produces acetaldehyde. The
cold-start problems have been addressed, and alcohol fuels are
technically viable, however with crude oil at
9.4 Why are CNG and LPG considered "cleaner" fuels.
CNG ( Compressed Natural Gas ) is usually around 70-90% methane with
10-20% ethane, 2-8% propanes, and decreasing quantities of the higher
HCs up to butane. The fuel has a high octane and usually only trace
quantities of unsaturates. The emissions from CNG have lower
concentrations of the hydrocarbons responsible for photochemical smog,
reduced CO, SOx, and NOx, and the lean misfire limit is extended [75].
There are no technical disadvantages, providing the installation is
performed correctly. The major disadvantage of compressed gas is the
reduced range. Vehicles may have between one to three cylinders ( 25
MPa, 90-120 litre capacity), and they usually represent about 50% of
the gasoline range. As natural gas pipelines do not go everywhere,
most conversions are dual-fuel with gasoline. The ignition timing and
stoichiometry are significantly different, but good conversions will
provide about 85% of the gasoline power over the full operating range,
with easy switching between the two fuels [76].
CNG has been extensively used in Italy and New Zealand ( NZ had
130,000 dual-fuelled vehicles with 380 refuelling stations in 1987 ).
The conversion costs are usually around US$1000, so the economics are
very dependent on the natural gas price. The typical 15% power loss
means that driveability of retrofitted CNG-fuelled vehicles is easily
impaired, consequently it is not recommended for vehicles of less than
1.5l engine capacity, or retrofitted onto engine/vehicle combinations
that have marginal driveability on gasoline. The low price of crude
oil, along with installation and ongoing CNG tank-testing costs, have
reduced the number of CNG vehicles in NZ. The US CNG fleet continues
to increase in size ( 60,000 in 1994 ).
LPG ( Liquified Petroleum Gas ) is predominantly propane with
iso-butane and n-butane. It has one major advantage over CNG, the
tanks do not have to be high pressure, and the fuel is stored as a
liquid. The fuel offers most of the environmental benefits of CNG,
including high octane. Approximately 20-25% more fuel is required,
unless the engine is optimised ( CR 12:1 ) for LPG, in which case
there is no decrease in power or increase in fuel consumption [17,76].
methane propane iso-octane
RON 120 112 100
MON 120 97 100
Heat of Vaporisation (MJ/kg) 0.5094 0.4253 0.2712
Net Heating Value (MJ/kg) 50.0 46.2 44.2
Vapour Pressure _at_ 38C ( kPa ) - - 11.8
Flame Temperature ( C ) 1950 1925 1980
Stoich. Flame Speed. ( m/s ) 0.45 0.45 0.31
Minimum Ignition Energy ( mJ ) 0.30 0.26 -
Lower Flammable Limit ( vol% ) 5.0 2.1 0.95
Upper Flammable Limit ( vol% ) 15.0 9.5 6.0
Autoignition Temperature ( C ) 540 - 630 450 415
9.5 Why are hydrogen-powered cars not available?
The Hindenburg. The technology to operate IC engines on hydrogen has
been investigated in depth since before the turn of the century. One
attraction was to use the hydrogen in airships to fuel the engines
instead of venting it. Hydrogen has a very high flame speed ( 3.24 -
4.40 m/s ), wide flammability limits ( 4.0 - 75 vol% ), low ignition
energy ( 0.017 mJ ), high autoignition temperature ( 520C ), and flame
temperature of 2050 C. Hydrogen has a very high specific energy (
120.0 MJ/kg ), making it very desirable as a transportation fuel. The
problem has been to develop a storage system that will pass all safety
concerns, and yet still be light enough for automotive use. Although
hydrogen can be mixed with oxygen and combusted more efficiently, most
proposals use air [73,77].
Unfortunately the flame temperature is sufficiently high to dissociate
atmospheric nitrogen and form undesirable NOx emissions. The high
flame speeds mean that ignition timing is at TDC, except when running
lean, when the ignition timing is advanced 10 degrees. The high flame
speed, coupled with a very small quenching distance mean that the
flame can sneak past inlet narrow inlet valve openings and cause
backfiring. The advantage of a wide range of mixture strengths and
high thermal efficiencies are matched by the disadvantages of
pre-ignition and knock unless weak mixtures, clean engines, and cool
operation are used.
Interested readers are referred to the group sci.energy.hydrogen for
details about this fuel.
9.6 What are "fuel cells" ?
Fuel cells are electrochemical cells that directly oxidise the fuel at
electrodes producing electrical and thermal energy. The oxidant is
usually oxygen from the air and the fuel is usually gaseous, with
hydrogen preferred. There has, so far, been little success using low
temperature fuel cells (
9.7 What is a "hybrid" vehicle?
A hybrid vehicle has three major systems [80].
- A primary power source, either an IC engine driven generator where
the IC engine only operates in the most efficient part of it's
performance map, or alternatives such as fuel cells and turbines.
- A power storage unit, which can be a flywheel, battery, or
ultracapacitor.
- A drive unit, almost always now an electric motor that can used as
a generator during braking. Regenerative braking may increase the
operational range about 8-13%.
Battery technology has not yet advanced sufficiently to economically
substitute for an IC engine, while retaining the carrying capacity,
range, performance, and driveability of the vehicle. Hybrid vehicles
may enable this problem to be at least partially overcome, but they
remain expensive, and the current ZEV proposals exclude fuel cells and
hybrids systems, but this is being re-evaluated.
9.8 What about other alternative fuels?
9.8.1 Ammonia
Anhydrous ammonia has been researched because it does not contain any
carbon, and so would not release any CO2. The high heat of
vaporisation requires a pre-vaporisation step, preferably also with
high jacket temperatures ( 180C ) to assist decomposition. Power
outputs of about 70% of that of gasoline under the same conditions
have been achieved [73].
9.8.2 Water
Mr Gunnerman has been promoting his patents that claim mixing one part
of gasoline with 2 parts water can provide as much power from an IC
engine as the same flow rate of gasoline. He claims the increased
efficiency is from catalysed dissociation of water to H2 and 02, as
the combustion chamber of the test engine contained a catalyst. It
takes the same amount of energy to dissociate water, as you reclaim
when you burn the H2 with 02. So he has to use heat energy that is
normally lost. He appears to have modified his claims a little with
his new A55 fuel. A recent article claims a 29% increase in fuel
economy for a test bus in Reno, but also claims that his fuel combusts
so efficiently that it can pass an emissions test without requiring a
catalytic converter [81]. Caterpillar are working with Gunnerman to
evaluate his claims and develop the product.
9.9 What about alternative oxidants?
9.9.1 Nitrous Oxide
Nitrous oxide ( N2O ) contains 33 vol% of oxygen, consequently the
combustion chamber is filled with less useless nitrogen. It is also
metered in as a liquid, with can cool the incoming charge further,
thus effectively increasing the charge density. With all that oxygen,
a lot more fuel can be squashed into the combustion chamber. The
advantage of nitrous oxide is that it has a flame speed, when burned
with hydrocarbon and alcohol fuels, that can be handled by current IC
engines, consequently the power is delivered in an orderly fashion,
but rapidly. The same is not true for pure oxygen combustion with
hydrocarbons, so leave that oxygen cylinder on the gas axe alone :-).
The following are for common premixed flames [82].
Temperature Flame Speed
Fuel Oxidant ( C ) ( m/s )
Acetylene Air 2400 1.60 - 2.70
" Nitrous Oxide 2800 2.60
" Oxygen 3140 8.00 - 24.80
Hydrogen Air 2050 3.24 - 4.40
" Nitrous Oxide 2690 3.90
" Oxygen 2660 9.00 - 36.80
Propane Air 1925 0.45
Natural Gas Air 1950 0.39
Nitrous oxide is not yet routinely used on standard vehicles, but the
technology is well understood.
9.9.2 Membrane Enrichment of Air
Over the last two decades, extensive research has been performed on
the use of membranes to enrich the oxygen content of air. Increasing
the oxygen content can make combustion more efficient due to the
higher flame temperature and less nitrogen. The optimum oxygen
concentration for existing automotive engine materials is around 30 -
40%. There are several commercial membranes that can provide that
level of enrichment. The problem is that the surface area required to
produce the necessary amount of enriched air for an SI engine is very
large. The membranes have to be laid close together, or wound in a
spiral, and significant amounts of power are required to force the air
along the membrane surface for sufficient enriched air to run a
slightly modified engine. Most research to date has centred on CI
engines, with their higher efficiencies. Several systems have been
tried on research engines and vehicles, however the higher NOx
emissions remain a problem [83,84].
Subject: 10. Historical Legends 10.1 The myth of Triptane
This post is an edited version of some posted after JdA posted some
claims from a hot-rod enthusiast reporting that triptane + 4cc TEL
had a rich power octane rating of 270. This was followed by another
that claimed the unleaded octane was 150.
In WWII there was a major effort to increase the power of the aviation
engines continuously, rather than just for short periods using boost fluids.
Increasing the octane of the fuel had dramatic effects on engines that could
be adjusted to utilise the fuel ( by changing boost pressure ). There was a
12% increase in cruising speed, 40% increase in rate of climb, 20% increase
in ceiling, and 40% increase in payload for a DC-3, if the fuel went from 87
to 100 Octane, and further increases if the engine could handle 100+ PN fuel
[85]. A 12 cylinder allison aircraft engine was operated on a 60% blend of
triptane ( 2,2,3-trimethylbutane ) in 100 octane leaded gasoline to produce
2500hp when the rated take-off horsepower with 100 octane leaded was 1500hp
[10].
Triptane was first shown to have high octane in 1926 as part of the General
Motors Research Laboratories investigations [86]. As further interest
developed, gallon quantities were made in 1938, and a full size production
plant was completed in late 1943. The fuel was tested, and the high lead
sensitivity resulted in power outputs up to 4 times that of iso-octane, and
as much as 25% improvement in fuel economy over iso-octane [10].
All of this sounds incredibly good, but then, as now, the cost of octane
enhancement has to be considered, and the plant producing triptane was not
really viable. the fuel was fully evaluated in the aviation test engines, and
it was under the aviation test conditions - where mixture strength is varied,
that the high power levels were observed over a narrow range of engine
adjustment. If turbine engines had not appeared, then maybe triptane would
have been used as an octane agent in leaded aviation gasolines. Significant
design changes would have been required for engines to utilise the high
anti-knock rating.
As an unleaded additive, it was not that much different to other isoalkanes,
consequently the modern manufacturing processes for aviation gasolines are
alkylation of unsaturated C4 HCs with isobutane, to produce a highly
iso-paraffinic product, and/or aromatization of naphthenic fractions to
produce aromatic hydrocarbons possessing excellent rich-mixture antiknock
properties.
So, the myth that triptane was the wonder anti-knock agent that would provide
heaps of power arose. In reality, it was one of the best of the iso-alkanes (
remember we are comparing it to iso-octane which just happened to be worse
than most other iso-alkanes), but it was not _that_ different from other
members. It was targeted, and produced, for supercharged aviation engines
that could adjust their mixture strength, used highly leaded fuel, and wanted
short period of high power for takeoff, regardless of economy.
The blending octane number, which is what we are discussing, of triptane is
designated by the American Petroleum Institute Research Project 45 survey as
112 Motor and 112 Research [31]. Triptane does not have a significantly
different blending number for MON or RON, when compared to iso-octane. When
TEL is added, the lead response of a large number of paraffins is well above
that of iso-octane ( about +45 for 3ml TEL/US Gal ), and this can lead to
Performance Numbers that can not be used in conventional automotive engines
[10].
10.2 From Honda Civic to Formula 1 winner.
The following is edited from a post in a debate over the advantages
of water injection. I tried to demonstrate what modifications would
be required to convert my own 1500cc Honda Civic into something
worthwhile :-).
There are many variables that will determine the power output of an engine.
High on the list will be the ability of the fuel to burn evenly without
knock. No matter how clever the engine, the engine power output limit is
determined by the fuel it is designed to use, not the amount of oxygen
stuffed into the cylinder and compressed. Modern engines designs and
gasolines are intended to reduce the emission of undesirable exhaust
pollutants, consequently engine performance is mainly constrained by the fuel
available.
My Honda Civic uses 91 RON fuel, but the Honda Formula 1 turbocharged 1.5
litre engine was only permitted to operate on 102 Research Octane fuel, and
had limits placed on the amount of fuel it could use during a race, the
maximum boost of the turbochargers was specified, as was an additional 40kg
penalty weight. Standard 102 RON gasoline would be about 96 R+M/2 if sold as
a pump gasoline. The normally-aspirated 3.0 litre engines could use unlimited
amounts of 102RON fuel. The F1 race duration is 305 km or 2 hours, and it's
perhaps worth remembering that Indy cars run at 7.3 psi boost.
Engine Standard Formula One
Year 1986 1987 1989
Size 1.5 litre 1.5 litre 1.5 litre
Cylinders 4 12 12
Aspiration normal turbo turbo
Maximum Boost - 58 psi 36.3 psi
Maximum Fuel - 200 litres 150 litres
Fuel 91 RON 102 RON 102 RON
Horsepower _at_ rpm 92 _at_ 6000 994 _at_ 12000 610 _at_ 12500
Torque (lb-ft _at_ rpm) 89 _at_ 4500 490 _at_ 9750 280 _at_ 10000
Lets consider the transition from Standard to Formula 1, without considering
materials etc.
- Replace the exhaust system. HP and torque climb to 100.
- Double the rpm while improving breathing, you now have 200hp but
still only about 100 torque.
- Boost it to 58psi which equals 4 such engines, so 1000hp and 500
torque.
Simple?, not with 102RON fuel, the engine would detonate to
pieces. so..
- Lower the compression ratio to 7.4:1, and the higher rpm is a big
advantage - there is much less time for the end gases to ignite
and cause detonation.
- Optimise engine design. 80 degree bank angles V for aerodynamic
reasons and go to six cylinders = V-6
- Cool the air. The compression of 70F air at 14.7psi to 72.7psi
raises its temperature to 377F. The turbos churn the air and
although they are about 75% efficient the air is now at 479F. The
huge intercoolers could reduce the air to 97F, but that was too
low to properly vaporise the fuel.
- Bypass the intercoolers to maintain 104F.
- Change the Air:Fuel ratio to 23% richer than stoichiometric to
reduce combustion temperature.
- Change to 84:16 toluene/heptane fuel, harder to vaporise, but
complies with the 102 RON requirement
- Add sophisticated electronic timing and engine manangement
controls to ensure reliable combustion with no detonation.
You now have a six-cylinder, 1.5 litre, 1000hp Honda Civic.
For subsequent years the restrictions were even more severe, 150 litres and
36.3 maximum boost, in a still vain attempt to give the 3 litre,
normally-aspirated engines a chance. Obviously Honda took advantage of the
reduced boost by increasing CR to 9.4:1, and only going to 15% rich air/fuel
ratio. They then developed an economy mode that involved heating the liquid
fuel to 180F to improve vaporisation, and increased the air temp to 158F, and
leaned out the air-fuel ratio to just 2% rich. The engine output dropped to
610hp _at_ 12,500 ( from 685hp _at_ 12,500 and about 312 lbs-ft of torque _at_ 10,000
rpm ), but 32% of the energy in the fuel was converted to mechanical work.
The engine still had crisp throttle response, and still beat the normally
aspirated engines that did not have the fuel limitation. So turbos were
banned. No other F1 racing engine has ever come close to converting 32% of
the fuel energy into work [87].
Subject: 11. References
11.1 Books and Research Papers
- Modern Petroleum Technology - 5th edition.
Editor, G.D.Hobson.
Wiley. ISBN 0 471 262498 (1984).
- Hydrocarbons from Fossil Fuels and their Relationship with Living
Organisms.
I.R.Hills, G.W.Smith, and E.V.Whitehead.
J.Inst.Petrol., v.56 p.127-137 (May 1970).
- Reference 1.
- Chapter 9. R.E.Banks and P.J.King.
- Ullmann's Encyclopedia of Industrial Chemistry - 5th edition.
Editor, B.Elvers.
VCH. ISBN 3-527-20123-8 (1993).
- Volume A23. Resources of Oil and Gas.
- BP Statistical Review of World Energy - June 1994.
- Proved Reserves at end 1993. p.2.
- Kirk-Othmer Encyclopedia of Chemical Technology - 4th edition.
Editor M.Howe-Grant.
Wiley. ISBN 0-471-52681-9 (1993)
- Midgley: Saint or Serpent?.
G.B.Kauffman.
Chemtech, December 1989. p.717-725.
- ?
T.Midgley Jr., T.A.Boyd.
Ind. Eng. Chem., v.14 p.589,849,894 (1922).
- Measurement of the Knock Characteristics of Gasoline in terms of a
Standard Fuel.
- Edgar.
Ind. Eng. Chem., v.19 p.145-146 (1927).
- The Effect of the Molecular Structure of Fuels on the Power and
Efficiency of Internal Combustion Engines.
C.F.Kettering.
Ind. Eng. Chem., v.36 p.1079-1085 (1944).
- Experiments with MTBE-100 as an Automobile Fuel.
K.Springer, L.Smith.
Tenth International Symposium on Alcohol Fuels.
- Proceedings, v.1 p.53 (1993).
12. Oxygenates for Reformulated Gasolines.
W.J.Piel, R.X.Thomas.
Hydrocarbon Processing, July 1990. p.68-73.
13. The Chemical Kinetics of Engine Knock.
C.K.Westbrook, W.J. Pitz.
Energy and Technology Review, Feb/Mar 1991. p.1-13.
14. The Chemistry Behind Engine Knock.
C.K.Westbrook.
Chemistry & Industry (UK), 3 August 1992. p.562-566.
15. A New Look at High Compression Engines.
D.F.Caris and E.E.Nelson.
SAE Paper 812A. (1958)
16. Problem + Research + Capital = Progress
T.Midgley,Jr.
Ind. Eng. Chem., v.31 p.504-506 (1939).
17. Reference 1.
- Chapter 20. K.Owen.
18. Automotive Gasolines - Recommended Practice
SAE J312 Jan93.
- Section 3.
SAE Handbook, volume 1. ISBN 1-56091-461-0 (1994).
19. Reference 6.
- Volume 12. Gasoline and Other Motor Fuels
20. Refiners have options to deal with reformulated gasoline.
G.Yepsin and T.Witoshkin.
Oil & Gas Journal, 8 April 1991. p.68-71.
21. Stoichiometric Air/Fuel Ratios of Automotive Fuels - Recommended
Practice.
SAE J1829 May92.
SAE Handbook, volume 1. ISBN 1-56091-461-0 (1994).
22. Chemical Engineers' Handbook - 5th edition
R.H.Perry and C.H.Chilton.
McGraw-Hill. ISBN 07-049478-9 (1973)
- Chapter 3.
23. Alternative Fuels
E.M.Goodger.
MacMillan. ISBN 0-333-25813-4 (1980)
- Appendix 4.
24. Automotive Gasolines - Recommended Practice.
SAE J312 Jan93.
SAE Handbook, volume 1. ISBN 1-56091-461-0 (1994).
25. Standard Specification for Automotive Spark-Ignition Engine Fuel.
ASTM D 4814-93a.
Annual Book of ASTM Standards v.05.03 (1994).
26. Criteria for Quality of Petroleum Products.
Editor, J.P. Allinson.
Applied Science. ISBN 0 85334 469 8
- Chapter 5. K.A.Boldt and S.T.Griffiths.
27. Meeting the challenge of reformulated gasoline.
R.J. Schmidt, P.L.Bogdan, and N.L.Gilsdorf.
Chemtech, February 1993. p.41-42.
28. The Relationship between Gasoline Composition and Vehicle Hydrocarbon
Emissions: A Review of Current Studies and Future Research Needs.
D. Schuetzle, W.O.Siegl, T.E.Jensen, M.A.Dearth, E.W.Kaiser, R.Gorse,
W.Kreucher, and E.Kulik.
Environmental Health Perspectives Supplements v.102 s.4 p.3-12. (1994)
29. Reference 23.
- Chapter 5.
30. Texaco to introduce clean burning gasoline.
Oil & Gas Journal, 28 February 1994. p.22-23.
31. Knocking Characteristics of Pure Hydrocarbons.
ASTM STP 225. (1958)
32. Health Effects of Gasoline.
Environmental Health Perspectives Supplements v.101. s.6 (1993)
33. Speciated Measurements and Calculated Reactivities of Vehicle Exhaust
Emissions from Conventional and Reformulated Gasolines.
S.K.Hoekman.
Environ. Sci. Technol., v.26 p.1206-1216 (1992).
34. Effect of Fuel Structure on Emissions from a Spark-Ignited Engine.
2. Naphthene and Aromatic Fuels.
E.W.Kaiser, W.O.Siegl, D.F.Cotton, R.W.Anderson.
Environ. Sci. Technol., v.26 p.1581-1586 (1992).
35. Determination of PCDDs and PCDFs in Car Exhaust.
A.G.Bingham, C.J.Edmunds, B.W.L.Graham, and M.T.Jones.
Chemosphere, v.19 p.669-673 (1989).
36. Volatile Organic Compounds: Ozone Formation, Alternative Fuels and
Toxics.
B.J.Finlayson-Pitts and J.N.Pitts Jr..
Chemistry and Industry (UK), 18 October 1993. p.796-800.
37. The rise and rise of global warming.
R.Matthews.
New Scientist, 26 November 1994. p.6.
38. Energy-related Carbon Dixode Emissions per Capita for OECD Countries
during 1990.
International Energy Agency. (1993)
39. Market Data Book - 1991, 1992, 1993 and 1994 editions.
Automobile News
- various tables
40. BP Statistical Review of World Energy - June 1994.
- Crude oil consumption p.7.
41. Automotive Gasolines - Recommended Practice
SAE J312 Jan93.
- Section 4
SAE Handbook, volume 1. ISBN 1-56091-461-0 (1994).
42. The Rise and Fall of Lead in Petrol.
IDG Berwick
Phys. Technol., v.18 p.158-164 (1987)
43. E.C. seeks gasoline emission control.
Hydrocarbon Processing, September 1990. p.43.
44. Health Effects of Gasoline Exposure. I. Exposure assessment for U.S.
Distribution Workers.
T.J.Smith, S.K.Hammond, and O.Wong.
Environmental Health Perspectives Supplements. v.101 s.6 p.13 (1993)
45. Atmospheric Chemistry of Tropospheric Ozone Formation: Scientific and
Regulatory Implications.
B.J.Finlayson-Pitts and J.N.Pitts, Jr.
Air & Waste, v.43 p.1091-1100 (1993).
46. Trends in Auto Emissions and Gasoline Composition.
R.F.Sawyer
Environmental Health Perspectives Supplements. v.101 s.6 p.5 (1993)
47. Reference 6.
- Volume 9. Exhaust Control, Automotive.
48. Achieving Acceptable Air Quality: Some Reflections on Controlling
Vehicle Emissions.
J.G.Calvert, J.B.Heywood, R.F.Sawyer, J.H.Seinfeld
Science v261 p37-45 (1993).
49. Radiometric Determination of Platinum and Palladium attrition from
Automotive Catalysts.
R.F.Hill and W.J.Mayer.
IEEE Trans. Nucl. Sci., NS-24, p.2549-2554 (1977).
50. Determination of Platinum Emissions from a three-way
catalyst-equipped Gasoline Engine.
H.P.Konig, R.F.Hertel, W.Koch and G.Rosner.
Atmospheric Environment, v.26A p.741-745 (1992).
51. Alternative Automotive Fuels - SAE Information Report.
SAE J1297 Mar93.
SAE Handbook, volume 1. ISBN 1-56091-461-0 (1994).
52. Lean-burn Catalyst offers market boom.
New Scientist, 17 July 1993. p.20.
53. Catalysts in cars.
K.T.Taylor.
Chemtech, September 1990. p.551-555.
54. Advanced Batteries for electric vehicles.
G.L.Henriksen, W.H.DeLuca, D.R.Vissers.
Chemtech, November 1994. p.32-38.
55. The great battery barrier.
IEEE Spectrum, November 1992. p.97-101.
56. Exposure of the general Population to Gasoline.
G.G.Akland
Environmental Health Perspectives Supplements. v.101 s.6 p.27-32 (1993)
57. Court Ruling Spurs Continued Debate Over Gasoline Oxygenates.
G.Peaff.
Chemical & Engineering News, 26 September 1994. p.8-13.
58. The Application of Formaldehyde Emission Measurement to the
Calibration of Engines using Methanol as a Fuel.
P.Waring, D.C.Kappatos, M.Galvin, B.Hamilton, and A.Joe.
Sixth International Symposium on Alcohol Fuels.
- Proceedings, v.2 p.53-60 (1984).
59. Emissions from 200,000 vehicles: a remote sensing study.
P.L.Guenther, G.A.Bishop, J.E.Peterson, D.H.Stedman.
Sci. Total Environ., v.146/147 p.297-302 (1994)
60. Remote Sensing of Vehicle Exhaust Emissions.
S.H.Cadle and R.D.Stephens.
Environ. Sci. Technol., v.28 p.258A-264A. (1994)
61. Real-World Vehicle Emissions: A Summary of the Third Annual CRC-APRAC
On-Road Vehicle Emissions Workshop.
S.H.Cadle, R.A.Gorse, D.R.Lawson.
Air & Waste, v.43 p.1084-1090 (1993)
62. IR Long-Path Photometry: A Remote Sensing Tool for Automobile
Emissions.
G.A.Bishop, J.R.Starkey, A.Ihlenfeldt, W.J.Williams, and D.H.Stedman.
Analytical Chemistry, v.61 p.671A-677A (1989)
63. A Cost-Effectiveness Study of Carbon Monoxide Emissions Reduction
Utilising Remote Sensing.
G.A.Bishop, D.H.Stedman, J.E.Peterson, T.J.Hosick, and P.L.Guenther
Air & Waste, v.42 p.978-985 (1993)
64. A presentation to the California I/M Review Committee of results of
a 1991 pilot programme.
D.R.Lawson, J.A.Gunderson
29 January 1992.
65. Methods of Knock Rating. 15. Measurement of the Knocking
Characteristics of Automotive Fuels.
J.M.Campbell, T.A.Boyd.
The Science of Petroleum. Oxford Uni. Press. v.4 p.3057-3065 (1938).
66. Standard Test Method for Knock Characteristics of Motor and Aviation
Fuels by the Motor Method.
ASTM D 2700 - 92. IP236/83
Annual Book of ASTM Standards v.05.04 (1994).
67. Standard Test Method for Knock Characteristics of Motor Fuels by the
Research Method.
ASTM D 2699 - 92. IP237/69
Annual Book of ASTM Standards v.05.04 (1994).
68. Preparation of distillates for front end octane number ( RON 100C )
of motor gasoline
IP 325/82
Standard Methods for Analysis and Testing of Petroleum and Related
Products. Wiley. ISBN 0 471 94879 9 (1994).
69. Octane Enhancers.
D.Simanaitis and D.Kott.
Road & Track, April 1989. p.82,83,86-88.
70. Specification for Aviation Gasolines
ASTM D 910 - 93
Annual Book of ASTM Standards v.05.01 (1994).
71. Reference 1.
- Chapter 19. R.A.Vere
72. Automotive Sensors Improve Driving Performance.
L.M.Sheppard.
Ceramic Bulletin, v.71 p.905-913 (1992).
73. Reference 23.
- Chapter 7.
74. Investigation of Fire and Explosion Accidents in the Chemical, Mining
and Fuel-Related Industries - A Manual.
Joseph M. Kuchta.
US Dept. of the Interior. Bureau of Mines Bulletin 680 (1985).
75. Natural Gas as an Automobile Fuel, An Experimental study.
R.D.Fleming and J.R.Allsup.
US Dept. of the Interior. Bureau of Mines Report 7806 (1973).
76. Comparative Studies of Methane and Propane as Fuels for Spark Ignition
and Compression Ignition Engines.
G.A.Karim and I.Wierzba.
SAE Paper 831196. (198?).
77. The Outlook for Hydrogen.
N.S.Mayersohn.
Popular Science, October 1993. p.66-71,111.
78. Reference 6.
- Volume 11. Fuel Cells.
79. The Clean Machine.
R.H.Williams.
Technology Review, April 1994. p.21-30.
80. Hybrid car promises high performance and low emissions.
M. Valenti.
Mechanical Engineering, July 1994. p.46-49.
81. ?
Automotive Industries Magazine, December 1994.
82. Instrumental Methods of Analysis - 6th edition.
H.H.Willard, L.L.Merritt, J.A.Dean, F.A.Settle.
D.Van Nostrand. ISBN 0-442-24502-5 (1981).
83. Research into Asymmetric Membrane Hollow Filter Device for OxygenEnriched
Air Production.
A.Z.Gollan. M.H.Kleper.
Dept.of Energy Report DOE/ID/12429-1 (1985).
84. New Look at Oxygen Enrichment. I. The diesel engine.
H.C.Watson, E.E.Milkins, G.R.Rigby.
SAE Technical Paper 900344 (1990)
85. Thorpe's Dictionary of Applied Chemistry - 4th edition.
Longmans. 1949.
- Petroleum
86. Detonation Characteristics of Some Paraffin Hydrocarbons.
W.G.Lovell, J.M.Campbell, and T.A.Boyd.
Ind. Eng. Chem., v.23 p.26-29. (1931)
87. Secrets of Honda's horsepower heroics.
C. Csere.
Road & Track/Car & Driver?, May 1991. p.29.
11.2 Suggested Further Reading
- Modern Petroleum Technology - any edition.
Editor, G.D.Hobson.
Wiley. ISBN 0 471 262498 (5th=1984).
- Hydrocarbon Fuels.
E.M.Goodger.
MacMillan. (1975)
- Alternative Fuels
E.M.Goodger.
MacMillan. ISBN 0-333-25813-4 (1980)
- Kirk-Othmer Encyclopedia of Chemical Technology - 4th edition.
Editor, M.Howe-Grant.
Wiley. ISBN 0-471-52681-9 (1993)
- especially Alcohol Fuels, Gasoline and Other Motor Fuels, and
Fuel Cells chapters.
- The Automotive Handbook. - any edition.
Bosch.
- SAE Handbook, volume 1. - issued annually.
SAE. ISBN 1-56091-461-0 (1994).
- especially J312, and J1297.
- Proceedings of the xxth International Symposium on Alcohol Fuels.
- Held every two years and most of the 10 conferences have lots of
good technical information, especially the earlier ones.
- various publishers.
- Alternative Transportation Fuels - An Environmental and Energy
solution.
Editor, D.Sperling.
Quorum Books. ISBN 0-89930-407-9 (1989).
- The Gasohol Handbook.
- Daniel Hunt.
Industrial Press. ISBN 0-8311-1137-2 (1981).
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